Friday, August 21, 2015

Already Raining in Eastern MA

It won't rain in Eastern MA until this afternoon (Me-yesterday).  Opps.  Well I blew that one.  Pouring in Middlesex, Suffolk,  and Essex counties.
NWS Northeast Regional Radar Loop
The combo surface and water vapor satellite shows the front and upper level low in the Northwest Atlantic nicely.  The low is undoubtably helping to feed moisture northward into Southeastern New England as it interacts with the cold front.
GOES_East Water Vapor, Surface 745 AM
Here is the 500 mb chart from 2 AM morning.  The upper level flow is into Quebec and North of Atlantic Canada.  Yet there is convergence from the interaction of the upper level features.  That puts New England in a favorable spot for heavy rain and thunderstorms today.  I should have seen this yesterday.
6z NAM 500 mb vort 2 am (mage NCEP)
That said the convergence at the upper levels should subside this afternoon, but a tropical air mass remains in place so thunderstorms capable of flash flooding and lots of lightning will be a threat all day.  The short range high resolution guidance suggests Eastern MA is under the gun for storms until later this afternoon before the bulk of the storms shift to Northern New England.  For example here is the simulated 8 PM radar.
11z HRRR simulated radar 8 PM (image NCEP)
WPC 1-3 day total rainfall forecast
Over the next 3 days the WPC thinks 1-2" of rain is likely
There will be more rain in areas where thunderstorms develop today.  There is a lot of moisture in the atmosphere.   Saturday looks to feature widely scattered showers and thunderstorms. 

-Zack Green

Thursday, August 20, 2015

Unsettled Weather for the Weekend

I've been stopped several times this week by people lamenting the forecast for rain all weekend.  Yes I expect some rain but not a washout.  I wouldn't recommend any camping trips to Northern New England but hold off on cancelling the cookouts and pool parties.  Summer is almost over so don't give up on a weekend so quickly.

General Overview
A strong trough of low pressure is slowly moving through the Great Lakes.  The system is trying to move towards New England but it is going to be forced up and over the ridge in the Western Atlantic Ocean.  Here is the upper level 500 mb chart.
12z NAM 8 am 500 mb vorticity, height (image NCEP)
Temperatures are very fall like in the Midwest.  Going back to 8 am lets see who can spot the cold front.
RTMA 2 m temperatures 8 am (image Weatherbell)
Fairly easy in this case.  As of noon the front has pushed a little further east and south, though not much.
WPC 8 am surface analysis
The water vapor satellite image shows the two major weather players for Southern New England this weekend and early next week.  A large upper level low pressure system is present near Bermuda.  This is forecast to slowly acquire tropical or subtropical characteristics over the next several days.  It may very well be named Erika before the weekend is over.  The cold front that will slowly approach should erode and stall somewhere in SNE this weekend and keep "Erika" out to sea but it may come close enough to enhance rainfall.  It will certainly enhance the surf at the shore so heads up if you plan to be in the water this weekend.
GOES_East water vapor satellite 1115 am (image NOAA)
Friday-Sunday
Precipitation ahead of the cold front works into Western New England before day break.  It has trouble getting into Worcester County during the day, but should have no trouble raining in Northern New England.  Friday night the front reaches Eastern New England where it stalls/begins to dissipate.  Temperatures are in the upper 70s/low 80s.  Lower temperatures where it rains a lot.
WPC surface forecast 8 PM Friday
 The hard part of the forecast is how much rain falls.  The subtropical connection off the coast should enhance the moisture flow into SNE.  We have a trigger to initiate convection (the front).  Locally 1-2"+ will fall.  Saturday appears to be the best day for SNE.  On Sunday Eastern MA, Cape and Islands will be at the highest risk for heavy rain as the low offshore makes its move north.  On Monday the storm will be at its closest point, but another cold front from the west will move to kick it out towards Atlantic Canada.

In summary periods of rain and thunderstorms is likely tomorrow through Monday.  Best chances for Western/Northern New England are Friday and Saturday, Southern New England on Sunday with isolated chances on Saturday.The dew points will be sticky as we are under the influence of a tropical air mass.  Speaking of the tropics...

Hurricane Danny
I had my doubts but the small compact nature of Danny has allowed it to strengthen into the first Hurricane of the 2015 season.  Given the overall tropical environment of the Main Development Region this is impressive.  The storm faces challenges as it heads towards the Caribbean Islands this weekend but so far it has shown it is a resilient tropical cyclone.
Hurricane Danny 11 am advisory (NHC)
 Winds are 75 mph and pressure is 992 mb.  It looks good on satellite
GOES_Floater visible satellite 1245 PM
Another tropical wave behind Danny has the potential to develop in the next 5 days.  By Sunday we could have Danny, Erika, and Fred.  Not bad for a strong El Nino year.

-Zack Green

Wednesday, August 19, 2015

Tropical Thoughts on Bob and Danny

Happy Hurricane Bob Day! For many it is a not so happy anniversary but it has been 24 years since the tropical cyclone stormed through Buzzard's Day with 105 MPH winds and 12-15 ft of storm surge.  Let's look back at Bob and take a look at Tropical Storm Danny way out in the Atlantic.  Please see my post from Saturday on my overall late August look at the tropics.

Hurricane Bob August 19, 1991
Bob developed in the Bahama's on August 16, 1991.  It was spawned by a cold front that exited the East Coast and degenerated.  The storm quickly intensified and moved north passing about 35 miles east of the Outer Banks.  The forward motion increased as Bob strengthened into a 115 MPH Category 3 Hurricane.  Cool ocean waters south of New England prevented Bob from keeping this strength (note- not the case this year see below).  Still on August 19 Bob roared into SE New England.   East of the center had very little rain but strong storm surge and strong Category 2 strength winds.  West of the center saw heavy rain.  This is the case with nearly all tropical cyclones due to the counter clockwise circulation.
NWS Boston Hurricane Bob graphic- from NWS Boston twitter feed
Although we faced considerable impacts from Floyd, Irene, and Sandy New England has not faced a direct Hurricane hit since Bob.  Are we due?  There is no such thing.  It all comes down to a combination of atmospheric/oceanic factors.  As you can see in my post from Saturday between 1938-1960 there were 5 direct Hurricane hits, not to mention the extreme flooding from Diane in 1955.  Even if the New England hurricane drought continues another 50 years we will always face a threat between August and October.

A 2013 report by insurance adjusters, in conjunction with the Great Hurricane of 1938's 75th anniversary, concluded that if Bob hit in 2013 it would have caused $35.4 Billion in damage, compared to $1.5 billion in 1991.  The coastline is super developed with valuable property.  Buzzards Bay is particularly vulnerable to storm surge.  Let's also consider that it took several communities 5-7 days to get power back from Irene.  What would a full fledged Hurricane do?

August 19, 2015 Tropical Update
Tropical Storm Danny formed yesterday in the open Atlantic Ocean.  Winds are currently 50 mph with a pressure of 1000 mb.
NHC 11 am est Advisory Tropical Storm Danny
The NHC thinks Danny becomes a Hurricane on Friday.  We'll see but the fact that the storm is small will help.  There are some unfavorable atmospheric factors in Danny's way.  Some guidance suggests these will subside.  First off on infrared satellite the storm does not have much deep convection with it (though it appears to have good structure).
GOES_East Rainbow IR satellite loop (NOAA)
The system may be pulling dry air into its circulation.   Also wind shear is evident in front of the storm.  The water vapor loop very dry air to the north along with strong wind shear.  This is typical of El Nino years in the Atlantic Ocean. 
GOES_East Water Vapor Loop (NOAA)
Let's see where Danny actually ends up in 5 days.  If it looks to be heading north of Puerto Rico it is something to really watch.  If it heads in the Caribbean I cannot see it surviving the hostile environment there.  The shear has been strong all season.  The storm looks to be small so the circulation would be ripped apart by the mountains.  But again if it can get in the SW Atlantic sea surface temperatures are favorable.
North Atlantic SST Anomalies (image Weatherbell)
I'll keep an eye on it.  Enjoy the cooler temps today and tomorrow.  Update on the weekend forecast plus Danny tomorrow.

-Zack Green

Saturday, August 15, 2015

Peak of Huricane Season Approaching, Any Action?

The vast majority of tropical cyclones that impact New England occur in August and September.  There have also been a few in October, most notably Sandy in 2012.  We have seen impacts in July and November as well (Arthur 2014, Noel 2007) but the highest potential for a strong storm is now. 
Personal Table of New England Storms of note
Irene was a tropical storm that dropped tremendous rain on Vermont and its winds knocked down thousands of trees and power lines in Eastern New England.  Power was out for 5 days for many areas in MA/RI.  That was eye opening as the past tells us much stronger storms have impacted the region.  These storms bring a variety of impacts to the region.  The Hurricane of 1938 is the GOAT as it hit without warning, at high tide, moving at 50+ MPH with sustained winds of 120 MPH.
Woods Hole, Hurricane of 1938 (image NOAA/NWS Boston)
Westerly, RI Brightmans Pond (Providence Journal, NWS Boston)
Here is the NWS Boston's overview of the storm put together in 2013 for the 75th anniversary of the storm.  The good news is that nothing in the Atlantic Ocean at the moment is even remotely capable for producing a storm like that.  In fact it has been a very quiet season with only 3 named storms.  Ana formed in May, Bill in June, and Claudette in July. 
2015 Atlantic Hurricane Season tracks (wikipedia)
All of the storms have formed close to land.  Why would that be?  In the Pacific Ocean a strong El Nino event has developed and that has enhanced tropical cyclone activity in that basin while suppressing activity in the Atlantic.  Here is the sea surface temperature anomaly chart.
OSPO Sea Surface Temperature Anomaly Chart 8-13-15
It's not hard to pick out the warmth in the Pacific.  While the Atlantic looks to be favorable strong winds aloft are ripping across the Caribbean Sea into the Eastern Atlantic.  Here is a graphic off the GOES_East satellite to illustrate the point (taken from the spaghetti model web page)
University of Wisconsin wind shear tendency chart
Although the wind shear relaxes a bit in the central Atlantic there is still dry, dusty air out there.  The dust comes from the Sahara Desert.
University of Wisconsin Meteosat-9 Saharan Air Layer
There are some waves trying to eat away at this dry air.  They will have a tough time but its possible one or two can form into a Tropical Cyclone.  In 1997 a major Hurricane was able to form in early September from a wave near the Cape Verde Islands.  1997 was the last time we saw an El Nino event this strong so it works a good analog. 
August 16, 1997 Sea Surface Temperature Anomaly (OSPO/NOAA)
Although there are differences, the general appearance remains the same.  Hurricane Bob was able form in 1991 during an El Nino.  It formed in the Bahama's and strengthened as it moved up the east coast. The weather pattern in week 2 appears to favor tropical development off the east coast and several global models and ensemble products indicate this.  I AM NOT PREDICTING A STORM LIKE BOB.  I am merely saying there could be "Danny" near the United States in the 6-10 day period.
RMOP Friday August 21, 8 PM
One thing that favors tropical development near the United States is a persistent ridge of high pressure in the NW Atlantic.  If some thunderstorms can get under this ridge they are usually blocked from moving anywhere too quickly.  Waters are warm near the East Coast so an area of low pressure forms and off we go.  The top analog according to the 6-10 upper air pattern is August 27, 1962.

CPC 6-10 upper air analog
We find Hurricane Alma forming in the Bahama's on August 26, 1962.
Hurricane Alma, August 26-30 1962
The 1962 Hurricane season was very quiet, much like 2015 thus far.  New England was also in a drought in 1962 so Alma brought a lot of beneficial rain to the region. 
Hurricane Alma rainfall (NHC/NOAA)
Of course the next analog on the list is August 13, 2002 and no storms are found in that time period.  The third date on the list is August 30, 1952.  Here we find Hurricane Able which formed from a tropical wave but did not really strengthen until it got under the ridge in the western Atlantic.
Hurricane Able August 18-September 3, 1952
In summary the Atlantic basin has been quiet which is consistent with strong El Nino events.  We are entering the time of year where New England sees its strongest tropical cyclones.  The week two upper air pattern seems to favor an atmospheric environment where a tropical system could (should?) form near the US.  Just something to keep an eye on.

For part 1 of today's post on the heat/humidity please click here.

Thanks for reading
-Zack Green 

Hot, Humid Weather for Next Several Days

The calender says mid August but it is still summer time.  The next week looks to be hot and humid with a few chances for thunderstorms.  Despite a soaking rain on Tuesday dry conditions have expanded again and these thunderstorm chances will not put a big dent into the moisture deficit.  The Atlantic basin remains quiet thanks it large part to El Nino, but also relatively cool sea surface temperatures in the tropical Atlantic and a drought in the Caribbean.  First up the upcoming heat and humidity.

General Overview/Short Term Forecast
A weak cold front will push through New England today but in general high pressure will be in charge.
8 am surface analysis (WPC)
Temperatures today should climb into the upper 80s/near 90.  Dew points will climb into the 60s in Southern New England.  The biggest threat for thunderstorms will start in Southern VT, NH, ME around 1-3 PM.  The threat shifts to Western MA around 4 PM and further east around 5-7 PM.  The steering in the upper atmosphere is weak so the biggest threat will heavy rain in a short period of time.  This may sound good because we are dry but this type of rain doesn't absorb into the soil as well as a steady soaking day long rain.
14z HRRR simulated radar 7 PM (image weatherbell)
Notice the dip in Northern Maine and the X in PA at 8 PM tonight .  This is the upper level support driving this system
12z NAM 500 mb vort 8 PM (image NCEP)
The shortwave will push offshore tonight and a ridge builds in for Sunday.  The tail of the front may trail near the coast tomorrow therefore we may have an isolated shower or storm tomorrow.  In general everyone stays dry however.  Here is the upper level chart
12z NAM 500 mb vort 2 PM Sunday (image NCEP)
With the trough not moving completely offshore I am expecting temperatures in the upper 80s tomorrow. It will be cooler at the Cape.  This is setting up a very warm Monday as the trough should push completely offshore by tomorrow evening.  Southwest flow will set enable a warmer, more humid air mass to invade next week.
WPC Surface forecast Sunday 8 PM
Monday highs should stay around 90, to perhaps as high as 94-95 in some spots like the Merrimack Valley, or Connecticut River Valley.  There is a low probability some could see isolated showers/thunderstorms in the PM.
Mid Range Outlook
A cold front will be approaching from the Midwest by Tuesday AM.  This will help to increase chances for showers and thunderstorms Tuesday into Wednesday though not a washout.  The front looks to slow down as it hits the Great Lakes.
WPC Tuesday 8 am surface forecast
A developing ridge in Southern Canada and the Northwest Atlantic will prevent the atmosphere from moving anything too quickly through.  It is not a very progressive pattern.  Picture it like the Mass Pike from Boston out to Framingham.  After the Weston Tolls 128/95 merges onto the Pike.  Then there is a lane drop another 1-2 miles up.  All the volume has to merge from 4 to 3 lanes and it just slows everything down.  That's the type of set up we are looking at.  There is good confidence in the block according to the relative measures of predictability (RMOP).
NCEP Ensemble RMOP Wednesday PM
The oranges in the middle of the US and in Atlantic Canada are showing high confidence in both the ridge being present in the Western Atlantic and the trough driving a cold front through the center of the country.  I would expect this ridge offshore to strengthen as we approach next week and into week 2.  That means any cold fronts will likely weaken and stall as they head towards the east coast.  That doesn't mean we see no rain but organized rain/storms will have a hard time getting towards the coast.  Northern and Western New England will have higher chances of rain.  Here is the drought monitor
CPC US Drought Monitor 8-11-15
The finer details can be sorted in the coming days.  If needed I will post a thunderstorm update for today at the top of the page later on.  See Part 2 for more on El Nino and the longer range tropical outlook.

-Zack Green

Monday, August 10, 2015

Rain, Thunderstorms for Tuesday

After nearly perfect weather Friday-Monday (my opinion) unsettled weather will move into Southern New England for Tuesday.  A widespread severe weather outbreak like last week doesn't appear likely.  However the rainfall will be widespread and embedded within the rain will be thunderstorms with isolated severe weather.  The Storm Prediction Center currently anticipates a marginal severe weather threat
SPC Day 2 Severe Weather Outlook
Here is the SPC outlook chart
SPC severe thunderstorm risk
Forecasting severe weather is hard.  The SPC does a great job overall but they sometimes struggle in New England as our severe weather outbreaks tend to be atypical.  That threat could jump to slight in the next update.    The system responsible for the active weather is currently in the Great Lakes
WPC 11 am surface analysis
In the upper levels the trough is rather shallow, with just a few shortwaves (at the surface the two low pressure systems to the North and South of Michigan) moving under a stronger shortwave in Ontario. 
12z NAM 500 mb vort valid 8 pm (image NCEP)
Temperatures are mild tonight, staying in the mid 60s as dew points and cloud cover increase.  By 6-8 am rain should be overspreading much of Southern New England from SW to NE.  I am anticipating two rounds of heavy rain, one for the early rush and one for mid afternoon.  Here is the 8 am projected surface chart
WPC 8 am projected surface chart
Southerly flow will allow for very moist, humid air to enter the region.  Upper level winds will also be potent for August so any thunderstorms will have the potential for damaging winds.  Still not expecting a lot of thunderstorms with the early rainfall.
12z hires NAM Precipitable Water forecast 2 pm (image weatherbell)
Pwats (precipitable water) are approaching and exceeding 2' in Southern New England.  The mid afternoon will be a period to watch as low pressure may form along the cold front as it pushes through.  That will enhance the thunderstorm threat for the 12-6 PM timeframe. 
WPC 8 PM Tuesday surface forecast
This projection would bring heavy rain/thunderstorms to the Cape and Islands tomorrow afternoon so heads up vacationers (let's see if my brother reads this, he is on the Cape this week).    The WPC is calling for 1.25-1.5' of rain regionwide and in general this will be correct.

WPC 1-2 day rainfall forecast through 8 PM Wednesday
Anyone who gets caught in one of the heavier showers/storms will be looking at over 2' of rain in a short period of time.  Areas prone to urban flooding will be difficult to navigate if this happens.  Guidance has been hinting at a more widespread thunder threat so I will monitor that threat and report an update tomorrow AM.

-Zack Green

Tuesday, August 4, 2015

BREAKING: Severe Thunderstorm Watch Issued for Much of New England

A wicked line of thunderstorms roared across Southern New England this morning downing trees, power lines, and ripping up front lawns.  Seriously
Cranston, RI via Hayleigh Meekers twitter feed
Rhode Island and SE MA look like they were painted blue because there were so many wind reports
SPC storm reports 8 am Mon 8 am Tue
Now another severe thunderstorm watch is up.
Severe Thunderstorm Watch til 8 pm
1 PM Update
The atmosphere is ready to explode this afternoon
CAPE 1 PM (image SPC)
 There is ample potential energy in and around Southern New England.  The winds are turning with height (ample wind shear)
Bulk effective wind shear 1 PM (image SPC)
These parameters (and many others) should lead to supercells and line segments alike this afternoon.  The radar has already popped
SNE Radar 110 PM (image NWS)
Keep an eye to the sky, especially North of MA Pike into S NH, VT, and ME.  The cold front will not pass until this evening so Southern New England (south of Pike, CT, RI) may get another round of storms later, in addition to anything the might pop this afternoon.
11 am surface analysis (WPC)
-Zack Green

Rain Ending As Snow For The Blackstone Valley Tuesday

Tomorrow is Tuesday and surprise, surprise...precipitation is in the forecast.  WBZ went back and found that there has been measurable preci...