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FXUS61 KPBZ 031729 AFDPBZ

Area Forecast Discussion National Weather Service Pittsburgh PA 129 PM EDT Fri May 3 2024

SYNOPSIS
Dry and unseasonably warm weather precedes increasing precipitation chances Friday afternoon and continuing through the weekend. Cooler, but still above average, temperatures prevail Saturday and Sunday with warm and unsettled weather continuing into next week.

NEAR TERM /THROUGH TONIGHT/
KEY MESSAGES:

- Showers and thunderstorms Friday afternoon and evening.
- Temperatures well above average approaching a few records.
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Did another minor refresh to the grids for today. Do think that the thickening cirrus will mute the temperature rise slightly today, and have 84-87 degrees as a top-end range for much of the area.
Although a shower or two may creep into the ZZV area this morning, the bulk of the shower/thunderstorm activity is still projected to kick off during the mid to late afternoon hours.

Previous Update

High pressure will retreat off to the east as a disturbance approaches from the west. Cirrus coverage will thicken and lower through the morning with increasing upper level moisture. Forecast soundings suggest another day similar to Thursday with a quickly breaking nocturnal inversion and a dry boundary layer allowing for mixing up to nearly 700 mb. The 850 mb ridge will gradually shift to the east and allow for warm advection in southwesterly flow to bring layer temperatures up to 15-16C across the area by afternoon, suggesting that high temperature will achieve the mid to upper 80s despite the cirrus. A couple sites have the potential to approach record highs (see climate section). Some lingering uncertainty in ensemble cloud coverage with suggestions for periods of scattering as cirrus push to the east lends potential for additional insolation to push temperatures up even a degree or two higher than currently forecast. This also could aid in a bit more destabilization ahead of showers and thunderstorms expected to develop by afternoon.

Indications are that showers and storms pop after 17z despite copious low level dry air (LCLs 6-8kft) as convective temperatures are met and a shortwave moves through, though weak net height rises through the day are expected. Mean hi res ensemble CAPE has increased in the latest run with 600-900 J/kg; conditional on potential scattering of clouds, the 75th percentile is up to 1400 J/kg. Most guidance suggests that warm mid-levels will suppress updrafts with an equilibrium level below about 500mb. Latest CAMs suggest that the best shot for deeper convection may be across eastern Ohio where slightly less warmth and moisture in the mid-levels may allow for greater vertical growth. Even if the higher end of the instability spectrum is achieved, weak cloud bearing layer flow will present a moderate CAPE/low shear environment likely more favorable of gusty wind with brief heavy downpours. PWATs will be pushing 1.5" and upwind propagation vectors around 10 knots despite practically no low level jet suggest this potential. HRRR probs for >0.5"/hr rates reach up to around 60% by evening. As a result, the Weather Prediction Center has our area in a Marginal Risk (1/5) for excessive rainfall on Friday.

Record low max temperatures may be broken Friday night with persistent cloud cover not allowing for radiative cooling.
Convection is favored to wane as instability is lost and the upper wave departs. Nonetheless, scattered showers will continue overnight.

SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/
KEY MESSAGES:

- Unsettled pattern remains with rain chances continuing through Sunday. Thunder possible on Sunday.
- Temperatures drop, but still remain above normal.
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Come Saturday, there is some suggestion the trough progression will come to a halt and allow rain to persist ahead of it being reinforced by approaching low pressure and additional shortwave support arriving from the south. The positioning of the trough will have a great influence on the weekend weather. No significant changes to the forecast for Saturday as instability looks limited to the western side of the stalled boundary/trough with a 50-60% of >500 J/kg while elsewhere plenty of low to mid level warm air will cap the environment. Total rainfall Saturday is favored in the half to three quarters of an inch range.

There is good agreement the trough finally pulls through by Sunday. An area of maximized precipitation ahead of or just along the trough axis is favored, as is a greater chance for more instability to work with. Ensemble probability for CAPE >500 J/kg reaches 70-80% Sunday, but with a good amount of clouds around and a low probability of any scattering. Still, with a bit better dynamics in play and marginal instability, a few thunderstorms accompanying the trough appear possible. Most likely total precipitation amounts Friday night - Sunday night sit around 1.0-1.3", but the 90th percentile exceeds 2" primarily across the WV panhandle and western PA resultant of some convective enhancement along the aforementioned trough.
This does not raise any flooding concerns for now, but may warrant watching if the higher end becomes more likely.

Thick cloud coverage through the weekend will hold temperatures in check, but still above average, in the 70s.

LONG TERM /MONDAY THROUGH THURSDAY/
KEY MESSAGES:

- Rain chances continue into early next week but more uncertainty lends lower confidence.
- Temperatures favored to remain above normal.
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After a brief lull in the precip late Sunday night into early Monday morning as flat upper ridging slides through and surface high pressure meanders on by, rain chances again increase. Seeing some better ensemble agreement for Monday with shortwave low pressure approaching from the southwest but weakening as the responsible upper wave flattens out. High pressure to the north should keep the highest precipitation chances south of I-70. Much more ensemble spread comes into play after Monday primarily with the progression of the upper pattern as ridging tries to develop but a deep upper low across the Dakotas throws shortwave energy to the east and tries to break the ridge down. A warm front likely lifts through sometime Tuesday into Wednesday increasing rain chances again, but with uncertainty in the strength and positioning of the ridge, timing and precipitation amounts remain low confidence at this time.

AVIATION /17Z FRIDAY THROUGH TUESDAY/
VFR will prevail this afternoon. High clouds will progressively thicken and high base cumulus fields may develop into the afternoon as heating continues. Guidance is consistent on developing shallow updrafts into the day today. Though there has been no observed lightning so far, the best heating has a chance of invigorating showers/storms into southwestern Pennsylvania and northern West Virginia. The current tempo groups highlight the period of best probability at the sites with the higher confidence in precipitation.

Into tonight, convection is expected to subside and become lower coverage before resolving to some morning rain.
Saturation from prior and current rain, with cooling temperatures may force a MVFR deck by daybreak, with probabilities of IFR greater than 50% at FKL and DUJ for now.
As the morning continues on, saturated surface conditions near the surface trough will allow rain and shower chances to continue into the afternoon in southeast surface flow with modest improvement in daytime heating.

Outlook
Restriction potential continues through Sunday morning, with additional storm chances Sunday afternoon. Retractions return early next week as multiple disturbances cross the region.

CLIMATE
Some record highs may be approached on Friday as well as record lows on Saturday. (* denotes current forecast temperature exceeds value)

5/3 Record High Year Pittsburgh 95 1887 Wheeling 90 1942 Morgantown 88 1965, 2012 New Philadelphia 89 2012 Zanesville 91 1938 Dubois 84 2012

5/4 Record Low Year Pittsburgh 65 1938 Wheeling 63 1931, 1939, 1941 Morgantown 63 2021 New Philadelphia 60* 2012 Zanesville 65 1902 Dubois N/A N/A

PBZ WATCHES/WARNINGS/ADVISORIES
PA...None.
OH...None.
WV...None.

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