Turner Downs Forecast Discussion

FXUS62 KRAH 200745

National Weather Service Raleigh NC
245 AM EST Thu Feb 20 2020

Low pressure will form off the Southeast coast late today, spreading
rain and snow across the area today into tonight. The storm will
move out to sea late tonight into Friday morning. Cold high pressure
will follow the storm system Friday and Saturday.


As of 200 PM Wednesday...

Changes: Still a good bit of uncertainty with respect to amounts,
though there will likely be a "sweet spot" and a tight gradient
somewhere across central NC. Primary changes for this forecast
issuance include an earlier start time for snow, particularly across
the northern half of the area and an uptick in amounts along the I-
85/US-64 corridors. While the overall weather pattern remains
similar to previous model runs, changes to the temperature profile
aloft have resulted in a bit more p-type uncertainty. There will
likely be a stripe of wintry mix across the area as the rain mixes
and transitions to snow across the area.

Overall pattern: A cold front will settle south of the area, laying
across SC/GA this morning with cool high pressure ridging southward
and advecting cold air into central NC. While the surface front will
remain south of the NC/SC border, albeit pivoting some to more SW/NE
orientation, the H85 front will stall over NC. As a s/w aloft rides
along the H85 front from the MS Valley toward the mid-Atlantic
during the day, a surface low will develop over the Gulf
Coast/Southeast US in response. With abundant moisture advection
aloft and and strong cyclogenesis at the surface off the Carolina
Coast there will likely be an area of strong frontogenesis over NC
during the aft/eve. The coastal low will lift northeast along the NC
coast then eastward out over the Atlantic through this eve/night.
Meanwhile, the parent upper trough will swing eastward through the
OH/TN Valley toward the mid-Atlantic/NE tonight. Precipitation will
quickly taper off as the upper trough swings through the area,
exiting the region early Friday.

Precipitation: Rain will move into the area from the south-southwest
early today. With the cool, dry air in place this rain will help
lower temperatures to the wet bulb temperature (which should be in
the 29-32 degree range), especially across the northern half of the
area. The tricky part of the precipitation will be determining the p-
type and transition times. There is a big potential for error in
this regard, but the most likely scenario will be a quicker
transition to snow across the north with lower liquid equivalent
precipitation totals than across the south. In between there will be
higher chances of mixed precipitation which could lower the overall
snow accumulation but potentially cause more issues travel-wise.
Based on the model simulations from this morning, there should be a
period of all snow, more brief in some areas than others, across
central NC between this morning and daybreak Friday. While generally
0.5" to 3" are most likely across central NC, the more realistic
scenario will be a very localized area of higher amounts and a tight
gradient from that to trace amounts. Of course, the thermal profile
and snow-liquid ratios will both impact the overall snow totals when
it`s all said and done. As per usual, the most significant
accumulations will occur on elevated/grassy surfaces.

Temperatures: Highs today will occur in the morning, topping out in
the upper 30s north to mid 40s south. Temperatures will lower with
the onset of rainfall, more quickly across the north with
temperatures in the mid 30s across the area expected by sunset.
Overnight lows will depend on how quickly the low moves away and the
precip ends, but generally expect a steady decrease to around
freezing until the rain ends, with more rapid cooling in its wake.
For now, expect temperatures could be in the mid 20s NW to upper 20s
SE by daybreak Friday.

Winds: Winds should be northeasterly throughout the period. While
sustained winds will remain in the 5-10 kt range in the NW, gusting
to 10-15 kts in the eve/night, winds across the southeast (in closer
proximity to the coastal low) will be quite a bit stronger and
gustier. Expect sustained winds across the Sandhills and Coastal
Plain around 15 kts with gusts into the mid 20s kts.


As of 245 AM Thursday...

Cannot rule out a snow shower remaining across the southeast early
Friday morning, but think the bulk of the precipitation will have
departed to the east. Expect that the bigger concern will be with
leftover liquid from today`s rain and snow freezing on area roadways
with morning temperatures in the 20s and lower 30s. Air temperatures
are not expected to rise above freezing until about noon. The bulk
of cloud cover will have also moved east of the region by sunrise,
and the day should be full of sun. With a stiff northerly wind, only
a few locations are expected to reach 40 degrees, with the bulk of
the area remaining in the 30s. Wind chills will generally remain in
the 20s. Winds will become light overnight as high pressure begins
to move into the region and the pressure gradient relaxes. Lows
everywhere will fall into the low to mid 20s.


As of 245 AM Thursday...

Saturday and Sunday should be beautiful days as high pressure moves
over the southeast United States. There will be little cloud cover
and highs in the 50s.

Cloud cover will dramatically increase Sunday night as low pressure
moves over the central Plains. Models have backed off the speed of
this system, so have also cut back on pops Sunday night - and may
not have been aggressive enough in reducing them. However, that low
will still be the major weather maker through the first half of the
week. The low will be near Cleveland by Tuesday morning and will
race northeast to Maine by Wednesday morning. The bulk of the rain
appears to be ahead of the front, which will pass over the Carolinas
on Tuesday. Have likely pops in the forecast Monday and Monday
night, dropping off to chance pops Tuesday before a brief respite
Tuesday night. Another weaker system is expected to travel across
the Appalachians Wednesday, bringing chance pops back into the
forecast. Temperatures will be a few degrees above normal.


As of 1240 AM Thursday...

VFR conditions are expected through 15z on Thursday. A mixture of
rain and snow will develop over the Piedmont between 15z and
18z/Thu, then transition to snow in the KGSO/KINT areas around 18z.
The rain/snow will transition around KRDU between 18z and 20z to all
snow. CIGS and VSBYS will dip into the IFR to LIFR category between
18z and 21z, lasting through 00z-03z, before the precipitation
tapers off. A return to VFR conditions is expected between 03z and
08z/Friday in the KINT/KGSO and KRDU areas. KFAY will have MVFR rain
conditions with CIGS/VSBYS Thursday - transitioning to snow by
between 00z-03z/Fri. IFR snow will last around KFAY and KFAY through
03-06z/Friday, before ending. VFR conditions will return on Friday.

Looking beyond 09z Fri, VFR conditions are likely to last Friday
through Sunday beneath high pressure. The next storm system will
arrive Mon, briefly a good chance of sub-VFR conditions with rain.


Winter Weather Advisory from 10 AM Thursday to 7 AM EST Friday
for NCZ007>011-021>028-038>043-073>078.



NWS RAH Office Area Forecast Discussion