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FXUS62 KRAH 161046

Area Forecast Discussion
National Weather Service Raleigh NC 
645 AM EDT Wed Aug 16 2017

A weak area of low pressure and a surface trough will linger near 
and off the Carolina coast through Wednesday night. A slightly drier 
air mass will move into the region on Wednesday. A back door cold 
front will drop into the region on Thursday and Friday before 
another cold front approaches from the northwest on Saturday. 


As of 205 AM Wednesday...

Pretty quiet day expected. Last evening's convection has culminated 
in a weak surface trough extending from coastal NC SW and W across 
northern SC, setting up a low level flow from the N over much of 
central NC today. The dry/stable trajectory is evident up through 
850 mb on the 00z upper air analyses, with NW flow at GSO and 
troughing over the eastern Carolinas, and warm/subsident mid levels. 
850 mb progs show the trough just aloft settling to our south with 
flow veering to northerly in eastern NC as a weak mesohigh forms 
over the NC Foothills and western Piedmont today. We'll likely see 
stratus and patches of fog this morning, thickest and most 
widespread across the northeast CWA beneath a focused 10-15 kts 925 
mb jetlet from the NE, although this feature will weaken and 
dissipate by afternoon, allowing for dispersion of the low clouds 
and a trend toward more sunshine. Expect partly to mostly sunny 
skies this afternoon, with minimal convection chances late in the 
day confined to the far S and W CWA where PW values should hold 
close to 2.0", while dipping to near 1.5" or lower elsewhere. Highs 
87-92 with slightly above normal thicknesses countering the morning 
clouds and delayed insolation. Expect dry weather this evening and 
tonight, although we'll need to watch for eastward drift of 
scattered convection off the higher terrain into the NW CWA 
overnight. Lows 71-76 under fair to partly cloudy skies. -GIH


.SHORT TERM /Thursday and Thursday night/... 
As of 220 AM Wednesday... 

Brief mid level shortwave ridging early Thu over the Southeast will 
give way to falling heights aloft and a WSW steering flow by Thu 
night, as potent northern stream shortwave troughing crosses the 
upper Midwest. Today's weak boundary to our south will be all but 
gone by Thu, leaving just a weak surface trough from western NC down 
through SC, and a light surface flow from the SE and S over NC. A 
subtle low-mid level warm frontal zone, seen on last evening's 700 
mb analysis extending from Nebraska through the MO Bootheel to MS 
and southern AL, will steadily progress to the NE and E, shifting 
across the interior Mid Atlantic and NC Thu morning through Thu 
evening. The presence of this deeper moisture, weak forcing for 
ascent, and daytime heating should prompt scattered showers and 
storms, spreading eastward through the day. Coverage should be 
slightly better across the NW CWA, closer to the lower mid level 
heights, as compared to the S/E CWA. Under partly to mostly cloudy 
skies, expect highs of 89-93 as thicknesses stay above normal. Pops 
should decrease in the evening with loss of heating and passage of 
the mid level moisture ridge to our east, but with such high surface 
dewpoints anticipated and the possibility of weak boundaries 
meandering about, will leave in a slight chance overnight. Lows in 
the mid 70s. -GIH


As of 300 AM Wednesday...

Friday and Friday night: An upper level shortwave trough and surface 
cold front will approach the region from the west on Friday, though 
it will likely get hung up over the mountains. As a result, strong 
southwesterly flow over Central NC will result in warm, humid 
conditions and the best chances for convection will be along 
differential heating boundaries near the foothills and along the sea 
breeze. For now, Friday night appears to be largely dry, although if 
enough pre-frontal moisture makes its way into Central NC, some 
convection could persist into the night. Highs in the upper 80s to 
mid 90s and lows in the low to mid 70s expected. 

Saturday and Sunday: A stronger upper level trough and surface cold 
front will approach from the west on Saturday, with the upper level 
trough swinging through the region and deamplifying as it does so 
Saturday night into Sunday. Although the front appears quite strong, 
it is expected to stall over Central NC, having a hard time 
progressing through the area before getting washed out Sunday night. 
Best chances for convection will be in proximity to the stalled 
front, lower in its wake, Saturday and Saturday night. Chances for 
convection could decrease further on Sunday, again with the highest 
chances farther southeast. Southerly to southwesterly flow at the 
surface will be enhanced ahead of the front with return flow around 
the Bermuda High extending into the region. Highs will generally 
remain in the upper 80s to low 90s over the weekend (although if the 
front progresses into the region a bit further it will affect temps 
to the northwest). Lows generally in the low to mid 70s.

Monday onward: Surface high pressure will build off the Mid-Atlantic 
coast, with continued southerly flow over Central NC through mid-
week. Aloft, high pressure will build over the Southeast U.S. with a 
trough amplifying over the Midwest Tuesday and progressing east-
southeastward. With the little change in airmass expected, continued 
diurnal convection in the favored areas will be most likely. As for 
temperatures, expect highs generally in the mid 80s North to low 90s 
South and lows in the upper 60s to mid 70s.


As of 645 AM Wednesday...

High probability of dominant IFR/LIFR conditions at RWI/RDU for the 
next few hours, with lower confidence and less coverage (if it 
occurs at all) at INT/GSO/FAY. A weak low pressure trough extending 
from eastern NC SW and W along the NC/SC border will become more 
defined today, with low level flow becoming uniformly from the 
NNE/NE. Latest high resolution models continue to support IFR/LIFR 
conditions over SE VA drifting to the ESE over northeast sections of 
central NC through 13z, followed by improvement to VFR by 15z or 
16z. RDU/RWI is likely to see the longest period of sub-VFR 
conditions early this morning. After 15z-16z, the low clouds will 
break up and lift, bringing VFR conditions to all sites, lasting 
through the end of the TAF valid period, with a couple of 
exceptions: There is a slight chance of a shower or storm, mainly 
impacting INT or FAY, from around 19z to around 02z, which may bring 
a period of sub-VFR conditions and erratic winds. And, IFR 
conditions are likely to develop at RWI in the last hours of the 
forecast period, after 06z. Winds will be light (under 8 kts) from 
the NNE or NE today, becoming variable under 4 kts this evening 
through tonight.

Looking beyond 12z Thu, IFR conditions are likely to hold into Thu 
morning in the NE (RWI), with just MVFR fog possible elsewhere. 
There is a good chance for showers and isolated storms both Thu and 
Fri afternoon, and sub-VFR fog/stratus are expected late Thu night 
into Fri morning and again late Fri night/Sat morning. Mainly 
afternoon and evening showers and storms are possible Sat through 
Sun as a cold front approaches from the NW. 






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