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FXUS62 KRAH 221921

Area Forecast Discussion
National Weather Service Raleigh NC
320 PM EDT Thu Mar 22 2018

Cool high pressure will build in from the northwest through Friday, 
before moving east and offshore Friday night. An area of low 
pressure will approach from the west Saturday and cross our region 
Saturday night, bringing a period of unsettled weather. 


As of 245 PM Thursday...

Through tonight: Mid cloudiness associated with the passing 
shortwave trough will gradually dissipate as the wave moves off the 
Carolina coast early tonight, as the surface ridge to our W/NW 
builds in. With overall sinking motion through the column and a 
light NW wind overnight, expect clear skies with decent radiational 
cooling. Lows 28-33.

Fri/Fri night: The mid level trough takes on a negative tilt out 
over the Atlantic Fri, keeping NC beneath northwesterly mid level 
flow as the surface ridge extends through the area. With a continued 
lack of moisture and descent through the column, expect fair to 
sunny skies Fri. Temps should top out a few degrees warmer than 
today, 53-60. A mid level shortwave trough over the Rockies Fri 
morning will top the ridge through Fri night, moving through the 
Midwest. An associated surface low will move E over the central 
Plains, while the preceding southerly flow out of the Gulf will 
prompt deepening moist upglide to our west and NW. As this moist 
ascent spreads eastward late Fri night, low chance pops will spread 
into W NC, and based on the average timing from the models, will 
bring in isolated precip chances late. The vertical thermal and 
moisture profiles (including the near-surface wet bulb trace) 
suggest that this may fall as a little light snow or sleet in the 
Triad late Fri night at onset, but impacts will be minimal to 
nothing. Given that clouds will be increasing from the west 
overnight, temps in the Triad may level off at milder readings, such 
that even low level wet bulbs may be at or above freezing. Expect 
lows of 28-37 with increasing clouds. -GIH


As of 330 PM Thursday... 

A low pressure system forming over Missouri will drop southward 
through the Carolinas over the weekend bringing a fair amount of 
rain and possibly some winter P-Types to the northern portions of 
our forecast area. Synoptically this system is neither a true Miller 
A or B scenario as the low drops in from the northwest instead of 
from the southwest and true secondary low never forms west of the 
mountains. Most of the jet dynamics with this system remain well to 
the north of the area, but as the low comes through Early Sunday 
morning an upper level jet streak passes to the southwest which will 
help bring some lift to the area. There is high confidence that this 
event will bring a good amount of rain to the area through the 
weekend, which won't be favorable for any outdoor plans. At this 
time, models are predicting up to an inch of rain for this event 
with higher amounts across the northern half of the forecast area.

Winter P-Types: There are many factors that make this a low 
confidence event for winter p-types at this time. An analysis of 
surface wetbulb temperatures show period Saturday morning of below 
zero wetbulbs across the Triad and VA border counties. The best 
chance for some snow mixing in would be right around 12z but the 
precipitation may not actually be in the Triad yet at that time or 
just arriving. When it does, it will have a lot of dry air to 
overcome in the low levels making sublimation a possibility for any 
snow. By the time those surface layers moisten up, temperatures may 
be too high to support snow. At that point, expect much if not all 
of the precipitation on Saturday to fall as rain. Expect high 
temperatures in the low to mid 40s across the north with mid 40s to 
low 50s in the south. 

Sunday morning will be the best chances for some accumulating snow 
as thermal profiles are more supportive of it and moisture and cold 
air are in place simultaneously. The problem is that forecast 
soundings lose a lot of the lift at this time and moisture in the 
dendritic growth zone is drying out. That would favor more sleet 
than snow. Therefore have added very small accumulations north of I-
85 and east to Halifax of less than a half of an inch on Sunday 
morning. Otherwise this should be a rain or rain/sleet event for 
points south. Sunday, temps will be in the low 40s across the Triad 
for highs with closer to 50 degrees in the south.


As of 130 PM Thursday...

Mostly dry weather is expected during the beginning and mid part of 
the work week, although the clouds should hang on through at least 
Mon evening (and perhaps into Tue in the western CWA) as a semi-
anchored surface high, centered over SW Quebec into the Maritimes, 
ridges SSW through NC, locking the wedge air mass over central NC. 
Mid level ridging will steadily build over the eastern CONUS through 
mid week, between deep closed lows over the Desert Southwest and 
over the NW Atlantic, ensuring a lack of forcing for ascent and 
continued tranquil weather (albeit with periodic cloudiness). Model 
solutions start to diverge Wed/Thu with the GFS trending more 
progressive, weakening and lifting out the NW Atlantic low, which 
allows the E Coast ridge to weaken and move east. The ECMWF and 
Canadian keep the Atlantic low in play with a southward drift, which 
keeps the mid level ridge over the Southeast. The slower blocking 
pattern is preferred, which will mean a trend to increasing sunshine 
and warming temps starting Tue and especially Wed/Thu, when low 
level thicknesses will reach or exceed seasonal normals with a 
modifying surface air mass. -GIH


As of 1230 PM Thursday...

VFR conditions will hold through the next 24 hours. A deep trough 
along the Eastern Seaboard today will shift eastward this afternoon 
through Fri, with drying and sinking air through the column. An 
expansive surface high building in from the NW through Fri will 
ensure dry low level air within a downsloping low level flow from 
the NW. A few gusts to 15 kts are possible this afternoon, otherwise 
light NW surface winds are expected.

Looking beyond 18z Fri, VFR conditions will hold through Fri night, 
although an approaching storm system from the NW will result in a 
trend to MVFR Sat morning at INT/GSO/RDU, and Sat afternoon at 
RWI/FAY, with precip moving in from the NW. INT/GSO will likely 
trend to IFR Sat afternoon. Precip is expected to start off wintry 
at all sites except FAY Sat morning before trending to mostly rain. 
Low level wind shear may also be a concern Sat afternoon/Sat night 
at FAY. IFR conditions should dominate at all terminals Sat night, 
followed by a slow trend to MVFR Sun, although IFR cigs may 
redevelop late Sun night at INT/GSO and persist through Mon, while 
other sites trend to VFR Mon. A trend to VFR is expected at all 
sites Tue. -GIH





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