Uncertain Severe Weather Threat Today

I’m not overly enthused about the severe weather threat today. That said, there are several mixed signals. The key has been all along how much low level moisture we’ll see over Connecticut. Rich low level moisture means big storms while less moisture should effectively eliminate the threat for strong thunderstorms. Take this morning’s HRRR for example.


That’s some impressive drying!

Screen Shot 2016-06-07 at 6.13.25 AMThe SREFs on the other hand show much higher levels of low level moisture with dew points >60F and CAPE around and in excess of 1000 j/kg.

You can see here there’s decent clustering of the ensemble members with dew points in the 60s for BDL. No member is anywhere near the HRRR with dew points dropping into the 50s and 40s by afternoon.

The 6z NAM has also trended a bit drier and a bit less impressive.

So today will be all about watching how quickly the low level moisture moves out of the state. With strong wind shear any storms that develop could become severe with adequate CAPE. The best chance for severe weather today will be in eastern Connecticut, Rhode Island, eastern Massachusetts up through portions of New Hampshire and Maine.



Memorial Day Forecast

This weekend forecast has been a struggle! For the last few days we’ve been trying to pin down the location and strength of a backdoor cold front which is nearly impossible to do. This morning when I first checked the models I knew our forecast probably was going to have to change again. The reason? A tropical wave off the Bahamas moving toward the Carolinas.


Whether or not this tropical wave becomes Tropical Storm Bonnie it doesn’t really matter. The low is going to open a conduit for a plume of tropical moisture to stream north into southern New England on Monday. You can see that here looking at precipitable water or PWAT forecast for Monday morning on the GFS (the Euro is actually even more impressive).


So what happens with that plume of moisture? Well, as the tropical juice streams north there’s also a jet streak in southern Quebec as a shortwave trough approaches from the lift. In the right entrance region of that jet streak air tends to rise and that will be right over southern New England.


The European model shows over 1″ of rain on Memorial Day here in Connecticut. A washout. The GFS shows rain but not to that extent. The European ensembles have an approximately 50% chance of >0.25″ of rain on Monday. Decent odds.


The actual “numbers” being printed out by the models aren’t particularly important at this point. The setup being forecast is one that looks to me like a wet one. Saturday and Sunday should be much better days. 2 out of 3 for a holiday weekend ain’t bad for the time of year!

Remarkable April Cold

Almanac BDL Rare Occurrence

What a remarkably cold day. The high temperature at Bradley Airport on 4/4 was 27F which was 29F below normal and was the second coldest April high temperature on record.

What I found most remarkable about yesterday’s chill was the zone of freezing rain that fell in southern Connecticut. As milder air moved in aloft cold air held firm near the surface -in fact for a period of time in the afternoon the temperature fell in New Haven as cold air drained south through the Quinnipiac River Valley.

KHVN 042353Z 01016G22KT 8SM BKN008 OVC021 M02/M04 A2995 RMK AO2 PK WND 36026/2329 SNB27E37 SLP141 P0000 60021 I1000 I6020 T10221039 10000 21022 53026
KHVN 042253Z 36016G22KT 10SM BKN008 OVC017 M02/M04 A2992 RMK AO2 FZRAE35 SLP131 P0001 I1003 T10221039
KHVN 042241Z 36014G21KT 10SM BKN008 OVC017 M02/M03 A2991 RMK AO2 FZRAE35 P0001 I1003 T10171033
KHVN 042153Z 01015KT 5SM -FZRA BR BKN008 OVC017 M01/M03 A2988 RMK AO2 SLP118 P0004 I1003 T10111028
KHVN 042053Z 36012KT 5SM -FZRA BR BKN009 OVC015 M01/M02 A2987 RMK AO2 SLP114 P0004 60016 I1006 I3014 T10111022 53008
KHVN 042025Z 01013KT 5SM -FZRA BR BKN009 OVC015 M01/M02 A2985 RMK AO2 P0001 I1003 T10061022
KHVN 042016Z 02013KT 6SM -FZRA BR SCT009 OVC015 M01/M02 A2984 RMK AO2 P0001 I1002 T10061022
KHVN 042009Z 01015G19KT 6SM -FZRA BR BKN009 OVC015 M01/M02 A2984 RMK AO2 CIG 008V012 P0001 I1001 T10061022
KHVN 042000Z 01011KT 6SM -FZRA BR SCT009 OVC016 M01/M02 A2984 RMK AO2 P0000 I1000 T10061022
KHVN 041953Z 02013G20KT 5SM -FZRA BR BKN009 OVC015 M01/M02 A2984 RMK AO2 SLP104 P0005 I1003 T10061022
KHVN 041940Z 01014KT 6SM -FZRA BR BKN009 BKN015 OVC049 M01/M02 A2985 RMK AO2 P0004 I1003 T10061022
KHVN 041932Z 01013G20KT 5SM FZRA BR SCT009 BKN015 OVC049 M01/M02 A2985 RMK AO2 P0004 I1002 T10061022
KHVN 041902Z 36010KT 4SM -FZRA BR BKN008 OVC015 M01/M02 A2985 RMK AO2 CIG 007V012 P0001 I1001 T10061022
KHVN 041853Z 36012KT 3SM FZRA BR BKN008 OVC015 M01/M02 A2984 RMK AO2 SLP104 P0007 I1005 T10061022
KHVN 041846Z 35010KT 4SM FZRA BR BKN008 BKN014 OVC031 M01/M02 A2984 RMK AO2 P0006 I1005 T10061017
KHVN 041813Z 35012KT 4SM -FZRA BR BKN008 BKN014 OVC031 00/M02 A2985 RMK AO2 P0003 I1003 T00001017
KHVN 041753Z 01009KT 4SM FZRA BR BKN008 BKN012 OVC041 00/M02 A2984 RMK AO2 RAE00UPB00E30FZRAB30 SLP106 P0012 60021 I1003 I6003 T00001017 10022 21006 58008

This was poorly forecast. All of our computer models showed warming in the boundary layer – and I certainly agreed with that. Additionally, getting snow or ice to accumulate on roads this time of year is difficult with the high sun angle and long days. Freezing rain in the middle of the afternoon in April – forget about it on the Connecticut shoreline! Even as of late morning yesterday the National Weather Service didn’t have winter weather advisories in effect for the four southern counties.

Check out the 13z HRRR forecast for New Haven. Only off by about 6C for the surface temperature!! The forecast for Bradley was better – though still off by several degrees Celsius.



The NAM was a bit better than the HRRR (though still too warm) but the NCAR ensemble may have been the winner of the day. The NCAR model did show the possibility of a really unusual April freezing rain event in southern Connecticut.


In fact, even this was underdone with 0.20″ of ice accretion measured by the ASOS at Tweed New Haven. Reports of sporadic damage with up to 1/3″ of ice accretion occurred a bit farther to the north.

Ice in Trumbull / Cindy Katske
Ice in Trumbull / Cindy Katske
Ice in Deep River / Hugh Albright
Ice in Deep River / Hugh Albright
Ice in Killingworth / @weather4life
Ice in Killingworth / @weather4life

This storm was a good reminder to not be too reliant on the HRRR. It’s tempting given its high resolution and hourly updates but in this case it really lead me astray. The NCAR ensembles were the way to go with this one – and I’ll give the NAM a shout out as well for doing much better handling the stubborn low level cold.

Sometimes the weather can care less about climatology – and this storm was one example. What is typically a slam dunk forecast in April – roads will generally be OK in the middle of the day with light (<1″/hour) rates was completely wrong in this case.

An unusual storm and a good learning experience!

Wild Sunday Morning Expected

Forecast Snow CT1122

Everything is coming together for an absolutely wild Sunday morning of weather across the state. Set your alarms for the show that will peak from 3 a.m. to 8 a.m..


An absolute nuke of a low pressure (compact but very intense) is expected to track across Long Island Sound. Most of our models show an exceptionally intense band of lift over Connecticut. Take for instance the NAM which shows nearly -70ubar/sec of lift right in the heart of the snow growth zone (-15c). What does that mean? An unusually efficient production of fluffy snow flakes will occur with exceptional snowfall rates. The RGEM (pictured above) goes nuts with the precipitation and in some parts of the state drops nearly 1.0″ inch of liquid!!!!


As the “nuke” passes by the barometric pressure will rise very quickly (>10 mb in 3 hours according to the NAM) which will lead to a brief period of ferocious winds. Gusts over 60 mph are possible.

Now, some of this will begin as rain and at least initially we will lose a bit to melting. That said, I do think there will be a narrow band of 1″-3″ of snow – but there is a shot that a small area (geographically) picks up 6″ of snow. This storm is so compact there will be many who are shut out from the storm but some who will get absolutely walloped.

Tough Storm Leaves – Finally

This storm was a real pain to forecast. Last week, most of our models had converged on a solution that indicated a major snowstorm was on the way. The GFS model, to its credit, did not.


The solutions on our computer models from Wednesday and Thursday were followed by much weaker and out-to-sea solutions with varying impacts for the state. Some models showed nothing and many showed a couple inches. By Friday we were confident that we wouldn’t see a blockbuster storm (>12″) but the final amounts were still up in the air.

As it turns out our forecast from Friday afternoon wasn’t a bad one.

Forecast from Friday at 5 p.m.
Forecast from Friday at 5 p.m.

On Saturday we pulled back the numbers a bit and then by Sunday morning basically brought them back to where we were on Friday. There’s nothing worse than a “windshield wiper” forecast but sometime it’s inevitable.

By the time all was said and done the storm didn’t actually produce a whole lot of precipitation in Connecticut – generally under 0.3″. In general that’s a pretty minor event. This storm was able to produce somewhat more impressive snow totals due to how fluffy the snow was. This was a very low density snow. In West Hartford the 3.4″ of snow I measured only contained 0.21″ of water (a 16:1 snow the liquid ratio). In Bakerville coop observer Denis Miller had a 23:1 ratio with 2.3″ of snow containing 0.10″ of liquid.

Snow and liquid totals from CoCoRaHS observers. Reports after 7:00 a.m. are close to final totals.
Snow and liquid totals from CoCoRaHS observers. Reports after 7:00 a.m. are close to final totals.

Why was this?


This is a forecast sounding this morning off the NAM for Hartford. Notice the white line that juts to the left? That farther left the faster the air is rising. It’s also happening at 12,000 feet above the ground where the temperature is -15C. This is where snow flakes form most efficiently and dendrites (those beautifully ornate crystals) are the favored crystal type. The combination of efficient snow growth and the presence of dendrites allows the snow to pile up quickly. Dendrites accumulate with a lot of air in between them which leads the snow to be low density. Temperatures near the ground were close to 32F which causes the snow to be tacky or sticky which is why it clung so readily to trees.

There’s a lot that goes into forecasting snow totals – figuring out the density of the snowfall is one of the most important things. Ironically, the snow that fell on the first full day of spring was the most fluffy snow I had all season in West Hartford. Strange indeed.