textproduct: Raleigh

WHAT HAS CHANGED

As of 425 PM Saturday...

* There is a Marginal risk (low probability and coverage) for severe storms Sunday afternoon through Monday morning

* There is an Enhanced Risk for severe storms Monday (higher coverage than usual and with isolated instances of significant severe, eg. EF2 or stronger tornado and 75 mph or greater gusts), with timing of greatest risk between 10A-7P. Residents should act this weekend to implement their severe weather preparedness plans and ensure means to receive severe weather alerts. More info at www.weather.gov/rah/severeprep

KEY MESSAGES

As of 425 PM Saturday...

1) Marginal risk (low probability and coverage) for shallow, rotating cells capable of an isolated spin-up/tornado or strong to locally damaging wind gust Sunday afternoon through Monday morning

2) Enhanced Risk for severe storms Monday (higher coverage than usual and with isolated instances of significant severe, eg. EF2 or stronger tornado and 75 mph or greater gusts), with potential for mixed mode (discrete or semi-discrete supercells with all hazards ahead of a QLCS with widespread strong to locally damaging wind gusts and corridors of significant wind and/or mesovortex tornadoes) and with timing of greatest risk between 10A-7P.

3) Tuesday and Wednesday will be much cooler than normal across central NC, with highs running 1520 degrees below mid-March averages and lows Tuesday night dropping into the 20s.

DISCUSSION

As of 425 PM Saturday...

KEY MESSAGE 1... Marginal risk (low probability and coverage) for shallow, rotating cells capable of an isolated spin-up/tornado or strong to locally damaging wind gust Sunday afternoon through Monday morning

A couple of shortwave troughs now over the nrn Great Basin/nrn Rockies and cntl AB/SK, respectively, will become increasingly- phased as they amplify a synoptic trough sharply across the Plains and MS Valley Sun-Sun night. Standardized 500 mb height anomalies within the trough are forecast to be negative 4-5 sigma over the lwr/mid MS Valley by 12Z Mon. Preceding height falls and strengthening lwr/mid-tropospheric flow will result downstream of the trough and throughout much of the ern US, including into cntl NC late Sun and particularly Sun night.

At the surface, a ~1022 mb high now over ern NC will be overtaken by a stronger one that will strengthen from ~1025 mb over the Great Lakes this afternoon to 1030-1035 mb off the New England and nrn Middle Atlantic coast through early Sun, while extending swwd across the srn Middle Atlantic Piedmont. A warm front will strengthen on the periphery of the ridge and retreat nwwd across cntl NC, where ely/sely and onshore flow will favor modification of the antecedent continental/dry airmass and with surface dewpoint recovery into the 50s F Sun afternoon. Richer, more-maritime low-level moisture, along a separate frontal zone now extending across the ern Gulf and cntl FL peninsula, will be transported nwd and across the srn Middle Atlantic Piedmont and Coastal Plain Sun night-Mon morning, in response to the aforementioned strengthening wind fields/low-level jet.

Given the only modest low-level moisture recovery coincident with afternoon heating, instability will remain weak and confined primarily to the srn Piedmont and Sandhills; and point forecast soundings to varying degrees depict a warm/dry layer based around 700-800 mb. As such, any cells that develop in that environment, probably invof the warm front, will be shallow but modestly sheared amid veering wind profiles in that layer, with an associated conditional risk of at least transient mini-supercells capable of strong to locally damaging gusts and/or an isolated spin-up/tornado. The latter risk would be more likely toward evening, as LCLs lower with nocturnal cooling, and strengthening low-level wind fields yield stronger SRH. As cells progress nwwd across the nrn Piedmont through evening, they should become increasingly elevated and/or weaken, through associated rain may briefly diabatically-strengthen/ reinforce the (wedge) front as it slows over w-cntl NC/VA.

A couple of additional, low probability/risks of severe will exist through Mon morning from 1) scattered cells moving onshore over sern/ern NC and into the vicinity of RAH's srn-cntl Coastal Plain and 2) with a risk of other scattered cells more-broadly throughout cntl NC, amid warm/moist advection and beneath the strengthening height falls aloft. Each regime would pose a risk of isolated tornadoes and strong to locally damaging gusts.

KEY MESSAGE 2... Enhanced Risk for severe storms Monday (higher coverage than usual and with isolated instances of significant severe, eg. EF2 or stronger tornado and 75 mph or greater gusts), with potential for mixed mode (discrete or semi-discrete supercells with all hazards ahead of a QLCS with widespread strong to locally damaging wind gusts and corridors of significant wind and/or mesovortex tornadoes) and with timing of greatest risk between 10A- 7P.

An intense synoptic trough will pivot and lift across the TN/OH Valleys and Appalachians and Middle Atlantic Mon-Mon night. Strong, 50-150 meter mid-level height falls will result across cntl NC (strongest nw) through Mon evening, during which time mid-level flow will increase to around 100 kts in the right entrance region of an intense (~125 kt), 500 mb speed max.

At the surface, a QLCS will likely to be ongoing at 12Z Mon near the spine of the Appalachians, along or probably slightly preceding a strong, polar front. An effective cold front will likely result from the QLCS; and this boundary will probably serve as the wrn bound of severe risk, with weaker convection possible along the synoptic, polar/cold front during the evening. Timing of the QLCS will be most favorable for strongest diurnal heating from the ern Piedmont ewd, and especially in the Coastal Plain (I-95 corridor). Widespread, strong to locally damaging wind gusts will probably result along the length of the QLCS, with corridors of 55-75 mph and locally >75 mph in embedded bowing segments and/or mesovortices. The latter will also favor the development of isolated tornadoes, a couple of which could be strong. There is a lower predictability for discrete or semi-discrete supercells ahead of the QLCS and effective and synoptic cold fronts, also favored over the Coastal Plain of NC/VA. Should some occur, they would pose a risk of all hazards, including large hail and also a risk of a strong tornado or two.

It will turn sharply colder behind the polar front Mon evening- night, with CAA-driven lows in the upr 20s-mid 30s.

KEY MESSAGE 3... Tuesday and Wednesday will be much cooler than normal across central NC, with highs running 1520 degrees below mid- March averages and lows Tuesday night dropping into the 20s.

Deep mid/upper troughing will set up over the Eastern US on Tuesday and Wednesday, as Canadian high pressure moves NE from the lower MS Valley into the Mid-Atlantic behind Monday's cold frontal passage. This will result in temperatures that would be below normal for January, let alone mid-March. Tuesday's highs will be in the lower- 40s to 50 with blustery WNW winds gusting to 15-20 mph. Under mostly clear skies and light winds with the surface high moving over or just north of our region, Tuesday night's lows will drop into the lower-to-mid-20s, coolest in outlying spots. Wednesday's temperatures will be slightly milder, reaching the mid-40s to lower- 50s, and winds will be lighter. Lows Wednesday night will still drop into the upper-20s to lower-30s. Conditions will turn milder from Thursday into at least Saturday as the high moves to the NE and away from the area.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

As of 738 PM Saturday...

24 hour TAF period: High confidence VFR conditions will prevail tonight. Winds will become mostly east or southeast around 10 knots.

MVFR to IFR ceilings are expected to develop over the region between 15z and 21z Sunday. This occurs as a warm front approaches. Ceilings will lower to IFR or LIFR late Sunday and Sunday evening as the warm front moves in from the south. Surface winds will increase from the SE at 15-20kt with scattered MVFR vsbys in showers.

Outlook: IFR to LIFR conditions are expected with breezy to windy SE becoming S winds at 15-25kt late Sunday night. A few showers will be possible as the warm front moves into southern VA late. Monday, a strong cold front should bring gusty winds from the SW to 35-40kt in gusts with scattered strong to severe thunderstorms mid morning through mid afternoon, with a line of potentially severe storms in the afternoon. This line will sweep quickly across the state Monday afternoon and evening, with potentially dangerous wind gusts. It will be windy with clearing skies Monday, with VFR conditions expected to return.

RAH WATCHES/WARNINGS/ADVISORIES

None.