textproduct: Raleigh

WHAT HAS CHANGED

As of 345 AM Friday...

* Low temperatures on Sunday nudged up slightly across the western Piedmont

* Lowered dewpoints on Saturday afternoon

KEY MESSAGES

As of 345 AM Friday...

1) Scattered convection will accompany a cold front and pose a conditional risk of an isolated instance of severe hail and/or wind between 5 PM and 9 PM, after record heat today, followed by a higher probability of a ~5-6 hour period of anafrontal, stratiform rain early tonight.

2) Fire weather conditions remain a concern for today and especially Saturday.

3) Widespread frost conditions likely Sunday morning with localized freeze conditions possible.

4) A period of dry weather will return this weekend and continue through mid week, resulting in a prolonged period of below-average precipitation that will exacerbate drought conditions and sustain or prolong the abundance of pollen in the air.

DISCUSSION

As of 345 AM Friday...

KEY MESSAGE 1... Scattered convection will accompany a cold front and pose a conditional risk of an isolated instance of severe hail and/or wind between 5 PM and 9 PM, after record heat today, followed by a higher probability of a ~5-6 hour period of anafrontal, stratiform rain early tonight.

Initially quasi-zonal flow evident in GOES-E water vapor satellite and 00Z/27th upr air data over the CONUS mid-latitudes will undergo amplification through 12Z Sat, as a couple of shortwave perturbations now over ND and srn SK modestly amplify across the Great Lakes and nrn Middle Atlantic. Associated, increasingly- cyclonic flow over the Middle Atlantic will be preceded by a convectively-amplified ribbon of mid-level vorticity accompanying a large area of anafrontal precipitation and convection now stretching from srn New England wswwd across the OH and mid MS Valleys and cntl Plains, which will progress sewd and into the Carolinas and TN Valley tonight. The wrn-most vorticity maxima within that mid-level vorticity and moist axis, now over ern KS, is a minor shortwave perturbation and formerly compact, closed cyclone that stalled near the nrn Baja Peninsula on Tue, which is forecast to continue to deamplify as it progresses ewd and across the srn Middle Atlantic this afternoon-evening. During that time, 40-60 m/12hr mid-level height falls and forcing for ascent may be regionally maximized over VA/NC. Generally wly, 500 mb flow will also steadily strengthen over cntl NC through that time, to between 40-60 kts (strongest north). Lastly, an EML plume, centered in 00Z upr air data from the lwr OH and mid MS Valleys from ILN to SGF, will have advected ewd and will be in place over the srn Middle Atlantic and Carolinas this morning (likely to be well-sampled in 12Z GSO and RNK RAOB data). A related plume of steep, ~7-7.5C/km mid-level level lapse rates will reside over cntl NC ahead of the features noted above and probably cap the environment to deep convective development until focused, frontal forcing arrives and overcomes diurnally-weakening CINH late this afternoon.

At the surface, a couple of closely-spaced cold fronts along/within the aforementioned precipitation band will progress ssewd/swd, with the lead of which likely to progress swd and across cntl NC between 21-23Z across far nrn zones and ~03-04Z across srn ones. The front will supplant preceding Bermuda high pressure and an unseasonably warm and seasonably moist airmass over cntl NC characterized by afternoon temperatures and dewpoints of ~85-90F and mid-upr 50sF, respectively. Weak instability, mostly between 250-750 J/kg, will likely be realized with heating of that seasonably moist air and beneath the plume of steep mid-level lapse rates. Scattered convection will likely develop along the front; and an isolated cell or two may deepen sufficiently to interact with the aforementioned strong mid-level flow and vertical shear that will be characterized by relatively long and straight hodographs through the mid/upr- levels. A conditional risk of hail and/or locally damaging wind gusts would result, with the former supported by mid-level rotation and steep mid-level lapse rates and the latter supported by steep low-level lapse rates and 20-25 degree surface dewpoint depressions. That convection should be quickly undercut by the swd-advancing surface cold front this evening, with following anafrontal, stratiform rain and much cooler and gusty nly/nnely surface winds that will develop in the pos-frontal regime tonight. Storm total rainfall amounts are expected to range from around a tenth of an inch over the srn Piedmont to a quarter to half inch elsewhere, and with locally higher amounts where convection occurs and tracks.

KEY MESSAGE 2... Fire weather conditions remain a concern for today and especially Saturday.

Minimum relative humidity values today will drop into the 30-40% range this afternoon ahead of any southward advancing precipitation. In addition, wind gusts outside of convection will generally range between 15 and 20 mph with a few stronger gusts. Although these conditions do not support typical fire weather conditions/criteria, the combination of unseasonably warm temperatures, with notable dryness within 100-hr fuels may provide for some adverse fire weather conditions today. This will be further complicated and especially hazardous around any ongoing fires by an abrupt wind shift from the cold front, or convective outflow, as it shifts south through the forecast area this afternoon and evening.

The degree and location of wetting rain that occurs this afternoon and tonight will modulate fire weather concerns for Saturday. Still, surface dewpoints will drop into the teens to around 20 supporting RH values that will bottom out between 15 and 25% during the afternoon. Wind gusts may be a limiting factor with wind gusts peaking at around 20 mph, although the strongest and most persistent gusts will be across the southeastern and eastern areas where RH values will tend to be higher.

KEY MESSAGE 3... Widespread frost conditions likely Sunday morning with localized freeze conditions possible.

A chilly but modifying 1036mb Canadian high pressure system will shift southeast into our region and setup across eastern NC and VA around daybreak Sunday morning. Low level thickness values will modify from Saturday and range between 1305 to 1320m on Sunday morning. Latest guidance suggests that the surface high may be centered a little further east which could curtail the previously expected ideal radiational conditions across the Piedmont where profiles note the potential for a little mixing. Adjusted lows downward a bit from the NBM into the 29 to 35 range. These temperatures would be especially hazardous to any early budding plants/vegetation that are sensitive to the cold. The Frost/Freeze program for central NC will begin the morning of April 1st, at which time we will resume the issuance of Frost Advisories, Freeze Watches/Warnings. Until then, will continue to highlight this risk in the HWO and gridded weather forecast.

KEY MESSAGE 4... A period of dry weather will return this weekend and continue through mid week resulting in a prolonged period of below-average precipitation exasperating drought conditions and sustaining or prolonging the abundance of pollen in the air.

Mid-level ridging aloft and a surface high that moves east from the OH Valley and Mid-Atlantic to the western Atlantic will result in dry weather from this weekend through at least Tuesday or Wednesday. D2 (Severe Drought) conditions already cover most of central NC, and the lack of rainfall may worsen those conditions further, especially with the very warm temperatures expected next week. Fire weather may also be a concern even if RH's don't reach criteria, due to the very dry antecedent conditions and breezy SW winds each day from Monday through Wednesday. Some relief from the dryness may arrive by late week, but this will depend on the positioning of frontal systems. There is a better signal that a potentially wetter pattern may arrive during the Easter weekend.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

As of 1255 PM Friday...

A strong cold front is on-track to approach from the north and surge swd and across cntl NC between 20Z-02Z. The cold front, and possibly a pre-frontal/lee trough, will be accompanied by scattered convection that may produce strong and gusty surface winds and hail before being quickly undercut by the cold front and weakening. Mostly MVFR ceilings, stratiform rain, and also strong and gusty nly to nnely surface winds, will follow in the post-frontal regime this evening/early tonight. A band of low-MVFR ceilings may linger from near RDU to FAY and RWI Sat morning, before clearing shortly after 12Z Sat and quickly returning to VFR for the rest of the day.

Outlook: Patchy fog and/or stratus will be possible amid a return flow regime, and associated increase in low-level moisture over cntl NC, by Tue morning.

RAH WATCHES/WARNINGS/ADVISORIES

None.