textproduct: Raleigh

WHAT HAS CHANGED

* Further decreased temperatures within the CAD regime, especially across the Piedmont, on Tuesday through Thursday.

KEY MESSAGES

As of 325 PM Monday...

1) Potential for strong storms with heavy downpours today across mainly the north.

2) Additional localized heavy rain possible areawide on Tuesday.

3) Cold air damming will bring much below normal temperatures during mid to late week.

DISCUSSION

As of 325 PM Monday...

KEY MESSAGE 1... Potential for strong storms with heavy downpours today across mainly the north.

A cold front is currently analyzed over southern VA, with cool NE flow and low stratus to its north. Ahead of the front, heating and destabilization have occurred, with widespread temperatures in the upper-80s to lower-90s across central NC and MLCAPE in the 1000-2000 J/kg range, highest over the NW Piedmont. Scattered cumulus has begun to develop, with a few isolated showers already popping up over parts of the northern and western Piedmont. The deeper convection is located along a convergence axis over western VA/NC as a shortwave approaches from western KY.

High-res models are in good agreement that convection will continue to expand this afternoon, though how far SE it makes before nocturnal stabilization is still uncertain as latest runs of the HRRR and NAMNest keep the heaviest rain just north of our CWA. Where storms do occur, locally excessive rainfall will be possible, with the greatest threat near the NC/VA border. 12z HREF LPMM indicates potential for isolated 1-3"+ amounts in those areas, which makes sense given above-normal PW values of 1.6-2 inches and very weak flow and shear (0-6km mean winds only around 5 kts) resulting in very slow-moving storms and possible merging and training of cells. Given dry antecedent conditions, think any flash flooding would be isolated and mainly limited to urban areas.

As for the severe threat, SPC has areas from RDU north and west in a marginal (level 1 of 5) risk today. The main threat is damaging winds given steep low-level lapse rates and DCAPE as high as 900 J/kg over the NW Piedmont. Any severe threat should diminish after dark.

KEY MESSAGE 2... Additional localized heavy rain possible areawide on Tuesday.

Central NC will likely be positioned between the leading edge of the synoptic cold front in VA, later to be established as the CAD front, and convective outflow and/or diff heating boundary from Mon/Mon night convection. A moderately unstable air mass with weak shear, tall skinny CAPE, above normal PWAT, and an approaching shortwave trough from the Ohio Valley into the Southern Appalachians will all favor an increase in convective coverage with slow moving storms, that may persist into the overnight period, capable of torrential rainfall and increased flooding risk (level 2 out of 4 from WPC Excessive Rainfall Outlook).

KEY MESSAGE 3... Cold air damming will bring much below normal temperatures during mid to late week.

As the surface front settles S of NC, confidence is high in the development of a robust, classical cold air damming event this week. With an early-week rex block in place over the Upper Midwest and Manitoba/Ontario, mid level energy will undercut the blocking high, crossing IN/OH before deepening to a low over the Mid Atlantic region during the mid-late week. This will keep the surface frontal zone pinned to our S and SE, with steady advection of cool and low- dewpoint surface air from a fairly strong and anchored surface high centered over Quebec. This high will weaken as it slowly retrogrades to the WSW late Thu into Fri, drifting into the N Great Lakes and S Ontario in response to what is expected to be strong coastal cyclogenesis just off the Mid Atlantic and Northeast coast by Fri/Sat, but the circulation around such a low alone should maintain the wedge regime and low level CAA through at least Fri, despite the likelihood of further drying aloft potentially eroding much of the CAD stratus by late in the day.

The result will be multiple days of high temps 10-15 deg F below normal, including highs from the Triangle to the N and W in the 60s to around 70 each day Wed through Fri. These temps will be further held down by copious cloud cover and widespread overrunning rain, driven in large part by moist isentropic upglide focused at 295K- 305K from Wed morning through Thu night. In addition to the cool days, we're likely to see a few nighttime lows in the 40s near the VA border Thu night through Sat night. Rain chances are expected to dwindle Fri and remain low into the weekend, as low-level stability decreases with the weakening and retrograding surface ridge. But the potential for a baggy mid level trough or shear axis holding across the region along with a persistent low level flow from the N or NNW around the coastal low should result in highs remaining at least a category below normal Sat/Sun. Given that we've seen above to much above-normal temps for all but a couple of days so far this month, several days of below to much-below normal temps will be a welcome respite for many.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

As of 845 PM Monday...

Of most immediate concern is the presence of scattered to numerous storms over portions of central NC, including near RDU and RWI which will have a tempo period of MVFR to IFR conditions in storms with variable winds 15-20 kts gusting to 28-38 kts until 02z/03z. These storms may reach FAY by 03z, but this has a much lower confidence level and will be monitored. Otherwise, once the storms dwindle to patchy showers by 04z, confidence is high that widespread MVFR then IFR/LIFR conditions will spread in from N to S, reaching the 4 northern terminals between 04z and 06z and near FAY by 08z-10z. While MVFR to IFR vsbys should improve by early Tue afternoon, these sub- VFR cigs will dominate across the area through Tue, although most sites will trend up to MVFR cigs Tue afternoon, and FAY may see a brief improvement to VFR after 16z. Areas of light rain are also expected from late morning Tue through the end of the TAF valid period, with highest confidence at INT/GSO/RDU, and this may bring a period of MVFR vsybs. Outside of any storms, surface winds will be mostly from the E or NE under 8 kts through Tue morning, then up to 8-15 kts with a few 20 kt gusts starting midday Tue.

Looking beyond 00z Wed, confidence is high that cigs will drop back to IFR/LIFR at all TAF sites by 04z Wed, lasting through at least mid morning Wed, although sub-VFR conditions are likely to dominate at all central NC terminals through much of Thu, as cool moist northeast air flow dammed up against the mountains holds in considerable low clouds over the area. Chances for VFR conditions improve by Fri, and VFR conditions should then dominate Fri afternoon into Sat.

RAH WATCHES/WARNINGS/ADVISORIES

None.