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February Medium/Long Range Discussion


snowmagnet
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Morning AFD from LWX 

SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...

...Uncertainty plagues potential snow threat this weekend...

After a couple of very mild days in the 50s to perhaps 60s, an
Arctic front will cross the region late Saturday. An initial
wind shift is expected to cause a change in direction from
south to west during the morning, but the brunt of colder air
is unlikely to arrive until very late at night east of the
mountains. Cold air chasing precipitation behind a front is not
normally an ideal setup for heavy snow east of the Appalachians.
This is in part due to the drying effects of downsloping, not
to mention compressional warming effects of downsloping that
tend to delay temperatures dropping east of the higher terrain.
Without a strong frontal wave nearby to hold in moisture,
prospects for significant snow would be low. However, strong
lift in the mid levels near the ideal snow growth zone, and an
area of low pressure developing near the Carolina coast could
act in tandem to produce a period of accumulating snow late
Saturday night into Sunday.

Questions remain regarding the strength of the offshore low.
Its strength will depend on how the northern and southern
branches of the jet stream interact, and just how quickly
temperatures drop. A large ridge over the western U.S. is in a
favorable position for downstream troughing and low pressure
near the East Coast, with some ridging evident near Greenland;
the orientation of the latter is not quite in the most ideal
spot to hold or "block" a storm`s path keeping it close to the
Mid-Atlantic coast. That being said, energy in both branches of
the jet stream is rather potent, and if they interact or phase
quickly enough, a stronger storm closer to the coast is still
possible. This scenario is most evident in some of the GEPS
(Canadian Ensemble/CMCE) guidance from the 00z suite, with the
GEFS still bullish but somewhat less than its 12z cycle, and
the EPS (Euro Ensemble/ECE) flatter, weaker, and drier.

A consensus approach (i.e. NBM) to all of the above smooths out
the swings from cycle to cycle and differences between
individual model camps, resulting in a 40-50% chance of
measurable snow across much of the area east of the mountains
late Saturday night into Sunday morning. Snow may mix with
rain, especially at the onset. Most likely snowfall amounts are
currently projected at less than 2 inches for most areas, but
both a low end scenario of little to no snow /and/ a high end
scenario of several inches are still on the table.

Potential impacts from this system may linger into Sunday. More
on that follows in the Long Term section below.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
To conclude the upcoming weekend, the forecast remains unresolved
regarding the threat for wintry precipitation as alluded to
above. 00Z guidance and their ensembles paint a mixed picture
of how much, if any, snow will occur with the weekend system.
The 00Z GFS/GEFS mean have been the most bullish on the
snowfall potential given a more favorably placed surface wave
and associated mid/upper forcing. Meanwhile, the 00Z ECMWF and
its ensemble mean are flatter with little to nothing over the
region. Sandwiched in between would be the CMC solution. With
forecast highs in the upper 30s, some locations along the I-95
corridor may see rain mixing in with any snowflakes. Given the
inherent model spread, much can change in the next 24 to 48
hours, so continue to check back on the latest forecast at
weather.gov/lwx/winter.
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Mount Holly this morning.. lol looks like they decided to forego all the details for what probably amounts to a minor event .

Not many changes were needed in the long term with only some subtle differences compared to earlier. After a spring-like day Saturday with very mild temps and a good deal of sunshine, a cold front crosses the area late. This will bring some sct showers and sharply colder air Sat night and into Sunday. The front only slows as it moves across the area so the showers may last into the overnight. A weak low develops on the front and brings more steady precip (mostly snow) to the lower Delaware Valley, Delmarva and much of NJ Sun morning and into the afternoon. A small accumulation is possible, but not certain at this point.

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Looking at the GFS over the past few model cycles, the primary NS shortwave has trended more towards a vorticity lobe(lacks sharpness) and the attempted phase with the southern shortwave is sloppy(partial/late).The precip we get as depicted is associated with the southern vorticity max that slides up ahead of the broadening trough as the NS dives in behind. This southern vort has trended sharper/ further SE. The NAM fwiw has it further NW.

Don't have time to go through all the guidance but suffice to say some changes in the subtle interactions are still possible.

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