textproduct: Raleigh

WHAT HAS CHANGED

* Updated the aviation discussion.

KEY MESSAGES

As of 425 PM Tuesday...

1) As a cold front approaches on Wednesday and Wednesday night, showers and storms will become more widespread across the northern Piedmont and northern Coastal Plain. Excessive rainfall and flash flooding will be possible, and Flood Watches may need to be issued. Locally damaging winds also can't be ruled out.

2) Widespread showers and storms with a risk for flooding will continue into at least Friday and possibly even this weekend, particularly in our south and east, as a cold front stalls near the NC/SC border.

DISCUSSION

As of 425 PM Tuesday...

KEY MESSAGE 1... As a cold front approaches on Wednesday and Wednesday night, showers and storms will become more widespread across the northern Piedmont and northern Coastal Plain. Excessive rainfall and flash flooding will be possible, and Flood Watches may need to be issued. Locally damaging winds also can't be ruled out.

Confidence is moderate-to-high for showers and storms to increase in coverage across the northern Piedmont late this evening into early Wednesday morning. A cold front pushing down slowly from Virginia will stall near the North Carolina border overnight, bringing a risk of continuous rainfall, which can be heavy at times, to the areas along and south of the NC border around daybreak. While an isolated severe storm with damaging wind gusts can't be ruled out tonight, the main concern will shift into Wednesday.

As mid/upper troughing with embedded shortwave impulses remains in place across the Eastern US on Wednesday, a strong cold front will approach and then slowly begin to cross the NW Piedmont on Wednesday night. While widespread cloud cover through the day will limit daytime heating across the north, models still show weak to moderate instability (up to 1000-1500 J/kg of CAPE) there. Above-normal PW values of 2-2.5 inches will also prevail ahead of the front, owing in large part to highly anomalous moisture being drawn in from the circulation of Tropical Storm Bertha over the northern Gulf.

CAMS generally show a band of showers and storms associated with a prefrontal trough that is mainly concentrated near the NC/VA border on Wednesday morning. This band will increase in coverage and intensity as it sags SE into the northern/western Piedmont and northern Coastal Plain in the afternoon and evening, continuing into the overnight hours. However, timing of this band is still somewhat uncertain, as it has slowed down in guidance like the GFS and NAM compared to the last couple days. If the 12z NAM Nest is accurate it won't really get going across our north until the evening and overnight hours, while the south stays mostly dry. So will have to watch is this trend continues.

Regardless, once the band does set up, excessive rainfall is looking increasingly likely given the very moist air mass and strong signal for storm training, as the mean flow is from the WSW (roughly parallel to the front). WPC QPF is in the 1-2.5 inch range across our far northern counties, but HREF LPMM depicts potential for 3-5+ inches in localized spots. Some of these areas have already received 1-3 inches over the last 2 days. Thus after internal collaboration with WPC, they are issuing a moderate (level 3 of 4) risk for excessive rainfall in our far NE Piedmont and northern Coastal Plain near the VA border for Wednesday and Wednesday night. Elsewhere, a slight (level 2 of 4) risk is in effect. Flood Watches may eventually be needed for parts of our area as well. Forecast rainfall amounts through Wednesday night quickly decrease to the south, with the 12z HREF mean depicting around a quarter inch or less in our southernmost tier of counties.

Furthermore, deep-layer shear of 30-40 kts across our northern counties could result in some organized storm cells/clusters that become strong to severe in the afternoon and evening. Thus most of central NC outside of the south is under a slight (level 2 of 5) risk for severe weather. Damaging winds will again be the main threat, and while 0-1 km SRH looks limited, an isolated tornado can't be totally ruled out.

KEY MESSAGE 2... Widespread showers and storms with a risk for flooding will continue into at least Friday and possibly even this weekend, particularly in our south and east, as a cold front stalls near the NC/SC border.

The cold front will move only slightly south and east on Thursday, and with rich Gulf moisture continuing to funnel NE along the front, another day of heavy rainfall and possible flash flooding is expected, this time across all of central NC. WPC QPF is 1-2 inches on average across the region, but this time the heaviest rainfall axis is shifted a bit SE. Once again, locally higher amounts will be possible. Thus after internal collaboration, WPC is also issuing a moderate (level 3 of 4) risk for excessive rainfall across eastern NC that includes some of our Coastal Plain counties, with a slight (level 2 of 4) risk for most of the rest of central NC.

This threat for heavy rainfall unfortunately looks to continue across at least our SE zones on Friday and possibly even Saturday as the cold front stalls or drifts very slowly south somewhere near the NC/SC border. WPC QPF from Friday into Saturday depicts another 1-3 inches from Raleigh south and east. There is still disagreement among the models on exact frontal placement, which will dictate where the heavy rainfall threat will be. However, guidance has generally been slowing down its southward progression and as a result keeps showers and storms across central NC for longer. Furthermore, a shortwave trough moving through the TN Valley and southern Appalachians may actually enhance the moist air mass again on Saturday as PW values increase to 2-2.5 inches, along with providing additional forcing for ascent.

One silver lining is the severe threat at least looks to decrease on Thursday and especially by Friday and Saturday. This is due to the Eastern US troughing flattening, resulting in a weakening of the mid- level flow, along with less instability to work with as the front slowly sinks south. However, a few strong storms with water-loaded downbursts still can't be ruled out mainly in the SE.

There is higher confidence in drier conditions from Sunday through Tuesday as the mid/upper troughing finally pushes to our east and the flow aloft turns northwesterly. However, as temperatures heat back up, isolated diurnally-driven convection still can't be ruled out each day.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

As of 150 AM Wednesday...

TAF period: Widespread VFR conditions across central NC early this morning are likely to continue through much of the overnight. A few rogue showers are possible across central NC although they will be isolated with much more widespread convection across VA, WV, and TN. Skies will generally be fair overnight with some SCT to possible BKN mid and high clouds. Southwesterly winds will continue overnight at 6 to 12 kts with gusts of 14 to 20 kts. A low level jet will strengthen overnight and some patchy LLWS is possible thru daybreak if surface winds relax but it appears that winds should stay up overnight for the most part and any LLWS will be patchy.

A cold front will sag south across the Virginias and Tennesse today and a region of showers and thunderstorms will shift south into central NC beginning around midday and slowly settling southward during the afternoon and evening. The convection will first impact the KGSO/KINT terminals between 16 and 20Z, KRDU/KRWI between 18 and 22Z and KFAY after 22Z. The convection will result in period of IFR VSBY restrictions, occasionally MVFR CIG restrictions and locally gusty winds. On and off showers and storms are apt to continue well into tonight with more widespread CIG restrictions spreading south across the area after midnight. Southwesterly winds at 10 to 14kts with gusts of 18 to 24kts are expected after daybreak through this afternoon with the winds relaxing a bit during the mid and late afternoon. Winds will shift to westerly and then northwesterly and decreasing further behind the front as it moves across the area tonight.

Outlook: Widespread showers and storms will result in adverse aviation conditions with VSBY and CIG restrictions on Thursday and Thursday night across central NC. The focus for the convection will shift slowly south into southern NC late Thursday and Friday with coverage decreasing notably and shifting further south on Saturday. Aviation conditions will improve on Sunday with a limited chance of showers.

RAH WATCHES/WARNINGS/ADVISORIES

None.