Day 2 Severe Weather Outlook (Friday 6/26/09)

Slight Risk runs south/east of line extending from roughly Roanoke VA-Culpeper VA-Washington D.C.-Baltimore MD-Poughkeepsie NY-Keene NH.

Cold front will continue to work eastward across the interior Northeast overnight Thursday into Friday Morning. Both NAM/GFS advertise moderate to locally strong destabilization taking place out ahead of the front along the I-95 corridor by early afternoon. Moderate Negative Lift Indices will be co-located in this axis of destabilization which should support gradual increase in thunderstorms by early afternoon. Despite low level shear being somewhat weak, potential will be in place for strong to severe storms, especially along/east much of the I-95 corridor.

..EGAN..

Day 1 Severe Weather Outlook (Thursday 6/25/09)...

D1 Outlook is now on the street. Most noticeable difference in latest model runs is to pull back on the timing with the frontal passage. This will keep the best forcing back thru interior PA/NY thru the afternoon into the Evening hours as front approaches from Southern Canada. It is here where Slight Risk was most justifiable at the present time. Slight Risk will run north/west of a line from roughly Morgantown-State College-Binghamton-Burlington. Pending on how things look heading into Thursday Afternoon, further adjustments may be needed with Midday update.

While MLCAPE and negative lifted indices will be favorable for thunderstorm development during the Afternoon and Evening hours across interior parts of NY/PA, weak low level vertical shear and wind flow will prohibit much in the way of organized and widespread convection. However, given the amount of cold air aloft, cells that do manage to develop will need to be watched for associated hail threat. This event could be very reminiscent to recent June 15th event.

Given this setup, expect most of the convection that does manage to develop to be of the Pulse-type variety, with large hail, damaging wind, and heavy rainfall to be the primary threats. With weak helicity in the lower levels, tornado threat appears rather low at the present time.

..EGAN..

12z Update on impending Winter Storm...

After one last review of 12z data, this will be Mike and I's final snowmap (2nd map overall). Only real significant change was to tweak the latter range. Upgraded the initial area of 12+ from locally higher (dashed hollow) to widespread (filled in) across eastern New England and issued a new locally higher area just downwind from this area that extends back to northeast suburbs of Philadelphia. In here, still expect amounts to be in the 8-12" range with highest amounts (10-12) on the eastern periphery (roughly from I-95 eastward). However, good chance some areas will see higher than 12" so hence area of locally higher that was inserted (dashed hollow). Outside of that, 1st map still looks in good shape, so no need to make any wholesale changes. I think the highest amounts end up falling in the Worcester Hills area of Northeast CT/Northwest RI/Central MA extending up into interior Maine. Localized amounts here may end up reaching 20".

...LTE

The Rare I-95 Snowstorm

It has seemed that in the most recent couple winters that it's been like pulling teeth to get a decent coastal storm into the I-95 corridor to dump a good heavy snowfall. Well take heart I-95ers... this ones for you.



With the 6-12 zone we expect lolipops of 15" in isolated spots possible and once into new England, some totals in the 12-18 range could show up in our dashed 12-18 zone. Remember our dashed zone still considers a general 6-12 but with the chance a bit more than the isolated spot gets a total in the 12-18 range. (This method worked out beautifully for the last coastal storm that killed NNE). We feel attm that has a better chance of happening further north.

Issues, track of course, that's going to setup the heavy snow axis and how far back the edge goes. We don't see mixing to be an issue for virtually anyone at this juncture, even if the storm ends up coming even further in the west. There is liable to be a tight gradient, perhaps not as tight as DT or the NAM would suggest but it will be a tight one. One significant thing i see at the moment is our current precip shield is way up into southern/southeast PA. Whether that's hitting the ground yet in some spots i'm not sure, but the precip is situated considerably further north than what the NAM, GFS, or ECMWF initialized. Couple that with the HPC notes about the GFS/NAM 500mb initialization and have to keep an open mind about the low being closer to the coast. Will reevaluate after 12z runs most likely.

Our models of choice right now is the GFS/ECMWF. the NAM just looks too tight and a bit east with precip IMO.

Big things i'll be watching for in the next couple model runs:
-Advisory-level snow penetrating west thru the PA I-81 corridor.
-A broader deform precip shield to the north and west.

...MAG

Another Signifcant Winter Storm on tap for parts of New York, New England...

First and likely final snow accumulations map covering the synoptic event combined with potential lake effect where applicable. This timeframe covers thru Monday Evening when the bulk of the action should finally be winding down, both with the synoptic event and expected Lake Effect event on the backside.

We could potentially see an expansion of 2-5" across the NY/PA/NJ corridor depending on how far south the secondary develops from what the model guidance shows. As of now, best chance for 2-4" will be along/east of I-81 and north of I-80 in Northeast PA northward into Central NY.

Across Northeastern New York and the northern half of New England, this is where the bulk of the snow is expected to fall. Current thinking is that most areas should stay at or under 20" from the synoptic event. That said, expectation is that there will be significant upsloping/terrain-induced snowfall following the end of the synoptic snowfall across the mountains/hills of Northeastern VT, Northern NH, and Western/Northern Maine that will carry on during much of the day on Monday. When all is said and done, this will result in total snowfall amounts reaching or exceeding 24" in localized areas. This is represented by the hollow dashed maroon zone of 20-30" which signifies an area where upwards of 20" is possible, but only localized and not widespread enough to fill in.

Concerning lake effect potential do, we have inserted a swath of 5-10" downwind of Lake Ontario with a localized highlight over parts of Oswego/Oneida/Madison/Onondaga counties where amounts could exceed 10". This is signified by a dashed hollow purple zone corresponding to the purple range of 10-20" seen in the legend. Again, dashed and unfilled signifies local amounts expected within that range, but not expected to be widespread enough to fill in.

...LTE

Significant Winter Storm Expected Across Parts of NY and New England

For the most part, this will just be a nuisance type storm with snow changing to rain. Most will only see a Trace to perhaps as much as 3" across the higher terrain. Across parts of Upstate NY into Northern New England, this event will produce a moderate to locally heavy snowfall, with highest amounts across the mountains. We have capped the snowfall at a maximum of 10", which will be possible across Adirondacks of New York, Green/White mountains of Vermont, and the Foothills of Northern New Hampshire and Maine.

We have highlighted several areas where locally heavier amounts are possible. Most of this area is in responses to the expected wrap-around and Lake Effect/Enhancement from Lakes Erie and Ontario on the backside of that Low Pressure. The region across North Central PA and the Northern Pocono mountains are where local amounts of up to 3" could be found from the intial wave of snow. Elsewhere, downwind of Lakes Erie and Ontario could potentially see some moderate to locally heavy amounts of snow, particularly the mountains of Northwestern Pennsylvania and the Chatauqua Ridge of Western New York. Concerning Lake Ontario... expect Oswego, Oneida, Lewis, and southern Jefferson counties will be under the gun for this event, with locally in excess of 1 foot possible.

We do expect a Lake Effect Event to setup during the day late Thursday into most of Friday, however this forecast package extends only thru Midnight Friday. Therefore, the projected accumulations are only meant to capture the early going enhancement before any significant Lake Effect Plumes set in.

LTE

Yet Another Messy Storm for the Northeast

While we are definitely not ruling out the possibilty of some icing in certain areas we think that it will be relatively minor. The atmospheric temps are going to support more of a snow vs rain scenario. With marginal surface temps and the warming aloft, I'm thinking rain and 33-35F is in the cards for many people after some initial snows.

For the transition zone we finally settled on just calling it snow/rain rather than snow to rain. It makes the map a bit more universal and general, a lot of folks are going to mix with and/or change to rain, and some are going to go back to snow. So instead of highlighting about 15 different scenarios, we kept it simple.

...MAG