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FXUS62 KRAH 231207

Area Forecast Discussion
National Weather Service Raleigh NC
807 AM EDT Mon Oct 23 2017

A potent upper level trough and accompanying surface frontal system 
will approach from the west and cross the Appalachians and middle 
Atlantic states this afternoon through tonight. A secondary cold 
front will follow and cross our region late Tue and Tue night. 


As of 415 AM Monday...

WV imagery this morning indicates a potent closed mid level 
low/shortwave trough has begun to assume a negative tilt from srn 
MO/nrn AR sewd into srn MS. This feature, and associated preceding 
50-100 m mid level height falls over the srn middle Atlantic states 
this aft-eve, is forecast to lift and accelerate newd through the OH 
valley and srn-cntl Appalachians through early tonight, before 
becoming absorbed by early Tue by a larger-scale trough/developing 
closed mid-upper low amplifying from s-cntl Canada to the Great 

At the surface, a 1010 mb surface low now over sern IL will rapidly 
deepen while migrating nnewd, to around 980 mb by the time it 
reaches the vicinity of nrn MI 24-30 hours from now. A trailing cold 
front now stretching swd from that low, through w-cntl TN and AL, 
will sweep ewd into the wrn Carolinas by this evening, then into the 
ern Carolinas by Tue morning.  Secondary cyclogenesis now underway 
over nrn AL will continue to occur at the intersection of this cold 
front and a warm front extending sewd from this secondary low, 
around the rim of the srn Appalachians/n-cntl GA enewd to nern 
SC/sern NC. This secondary low will migrate newd through the wrn 
Carolinas, along the retreating warm front and what will likely 
become an in-situ wedge front as early day showers overspread the 
wrn Carolinas and interact with a lingering continental air mass 
from the high pressure ridge that has been in place in recent days. 

An axis of low clouds and showers now stretching across SC will 
pivot newd across the srn and wrn NC Piedmont this morning; and this 
should serve to hold temperatures aob 70 degrees over the wrn 
Piedmont today, and lead to the development of the aforementioned in-
situ wedge front. Meanwhile, multi-layered cloud cover will increase 
across the remainder of cntl NC, but with peaks of at least filtered 
sunshine that should allow temperatures to climb into middle 70s to 
around 80 degrees - similar to those of recent days. 

Strong and focused forcing for ascent accompanying both the 
negatively-tilted trough aloft and the surface front will likely 
result in a strongly-forced QLCS, possibly preceded by a warm 
frontal band of scattered convection, which will spread east across 
the srn/wrn Piedmont between 4-8 PM, then ewd across the Triangle 
and surrounding areas between 6-10 PM, and lastly across the 
I-95 corridor/Coastal Plain between 9 PM and 1 AM. 

Instability will be maximized over the ern Carolinas, into perhaps 
the 500-1000 J/kg range, while the corridor of strongest forcing for 
ascent will translate through the Piedmont of the Carolinas and VA, 
where weaker instability on the order of several hundred J/kg will 
likely materialize. Though weaker, this buoyancy should still be 
sufficient to support a severe threat owing to at least moist-
neutral lapse rates in the lowest km, and more than sufficient lift 
to compensate for the relative minimum in instability. 

The degree and character of both low and deep layer shear will be 
more than supportive of organized storm modes, including supercells 
and bows/lewps --with isolated tornadoes-- embedded within what will 
otherwise be a strongly-forced QLCS that will pose a risk of strong 
to damaging wind gusts along its length, resulting from momentum 
transport of a 50-70 kt llj between 925-850 mb. That llj will also 
contribute to extreme low level shear characterized by 
enlarged/cyclonically-curved hodographs and associated 400-800 m2/s2 
of effective SRH, strongest over the Piedmont, where the risk of the 
aforementioned isolated tornadoes appears to be greatest.

Lastly, storm total rainfall amounts are expected to range from 
around one half to one and a half inches, highest over the wrn 
Piedmont, where both early day showers and the stronger forcing for 
ascent will favor the highest amounts. While these values are well 
short of 1-3 hr flash flood guidance values in the 3-4" range, some 
urban flooding will be possible, particularly around the Triad. 

Clearing behind the convective line, with the passage of the cold 
front, will spread west to east overnight. Areas of low clouds and 
fog may redevelop over the srn/wrn piedmont, where clearing will 
occur first, and lessening winds between this lead cold front, and a 
trailing one scheduled for a late Tue-Tue night passage, will favor 
modest-strong radiational cooling. Lows are expected to range from 
the upper 40s west to lower-mid 60s east. 


As of 310 AM Monday...

Tuesday morning, precipitation associated with a cold front moving 
through central NC will be in the vicinity of the eastern edges of 
the CWA and heading eastward with time. Since the low pressure 
system associated with this front will be located well to the 
northwest of the area over the Great Lakes, it will take some time 
for the colder airmass to filter in behind the front. Therefore high 
temperatures will remain fairly high on Tuesday with upper 60s to 
mid 70s expected across the area from NW to SE. Lows Tuesday night 
will be in the low 40s across the NW Piedmont to near 50 degrees in 
eastern counties.


As of 310 AM Monday... 

After the surface front moves through the area by Tuesday morning, 
attention will turn to the parent low pressure system which will 
hang back over the Great Lakes for a day or so before weakening 
considerably and heading due north into Canada. As a result of this, 
the upper level trough never really digs southward and upper level 
flow over central NC doesn't ever go northwesterly. While the 
surface air will certainly be cooler than we have been, it isn't an 
extremely cold airmass that will be filtering into the area. 
Therefore expect highs to drop into the upper 50s to mid 60s 
Wednesday and Thursday but then moderate once again to near 70 
degrees for the end of the work week. The coldest night will be 
Wednesday night but guidance shows only upper 30s to lower 40s and 
with some light westerly winds and possibly some clouds aloft, frost 
may not be a threat. Lows will remain in the 40s for the majority of 
the week and then climb back into the upper 40s to lower 50s for the 

The next frontal system will approach the area next weekend but 
there is uncertainty with regards to timing and also the evolution 
of a low pressure system riding up the southeast coastline prior to 
the arrival of the cold front that could cause a chance of 
precipitation earlier in the weekend.


As of 800 AM Monday...

In strengthening and increasingly moist east to sely low level flow 
off the Atlantic ocean, an axis of IFR-MVFR ceilings and showers 
will continue to move across wrn portions of the forecast 
area/mainly at Triad TAF sites this morning, then continue, with 
periods of IFR ceilings, today through this evening. 

Aside from a brief instance of an IFR-MVFR ceiling and/or a shower 
at RDU/FAY/RWI this morning, generally VFR conditions are expected 
to persist at those locations until this evening. 

An approaching frontal system will then result in the ewd 
progression of a band of IFR-MVFR ceilings, heavy showers and 
isolated storms with strong and gusty sly to sswly winds --including 
some severe with isolated tornadoes and straight line wind gusts aoa 
50 kts-- between 22Z/23rd and 05Z/24th. Sely surface winds will 
otherwise increase and become strong and gusty by late morning to 
midday, ahead of the expected line(s) of convection. West to east 
clearing will occur with the passage of the associated cold front 
this evening-early tonight, with a period of enhanced swly winds in 
the hour or three immediately following the frontal passage. 
Lessening winds and lingering low level moisture late tonight may 
result in the development of fog and low stratus primarily at INT 
and GSO between 08-12Z/24th, though the latest model guidance has 
backed off on this potential. 

Outlook:  VFR conditions are anticipated for at least the next 
several days. 





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