textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Added Coastal Monmouth to a Freeze Warning and updated timing of Freeze Warnings.

KEY MESSAGES

1. A widespread freeze is expected to occur tonight as an unusually cold airmass settles over the area.

2. A return to above average temperatures later this week before cooling down again over the weekend. Widespread rainfall possible over the weekend.

DISCUSSION

KEY MESSAGE 1...A widespread freeze is expected to occur tonight as an unusually cold airmass settles over the area.

In wake of departing cold front, a rather strong area of high pressure quickly builds over central Pennsylvania tonight. This high will cause northwest winds to diminish quickly through the evening hours and lead to an exceptional night for radiational cooling under clear skies. Not only will the airmass be unusually cold, but it will be quite dry with dew points falling into the low teens to low 20s. Under these atmospheric conditions, the boundary layer should decouple, so have opted to blend in some colder guidance, lowering lows for tonight into the low 20s to low 30s for areas away from the coast. Freeze Warnings remain in effect tonight, and have added Coastal Monmouth County into the Warning as well. For more information regarding daily low temperature records, please refer to the Climate Section below.

For remaining coastal locales, from Coastal Ocean to Coastal Sussex, thinking that may be enough of a marine influence to keep temps in these areas from falling below freezing. While some patchy frost is possible in these areas, it may not be widespread due to winds remaining elevated. Nonetheless, lows will fall into in the mid to upper 30s in these areas.

As a reminder, all of our counties now have an active growing season due to the recent excessive warmth. Blooms/vegetation have quickly initiated and in some cases are ahead of schedule. Fruit trees and other cold sensitive plants already planted are of the most concern. Frost formation might be somewhat limited given a dry low-level airmass in place tonight.

KEY MESSAGE 2...A return to above average temperatures later this week before cooling down again over the weekend. Some showers expected on Wednesday morning with more widespread rainfall possible over the weekend.

It is not anticipated to be as cold Tuesday night as the air mass continues to modify and clouds increase with high pressure now well offshore. A return, southerly, flow will also become established Tuesday and continue Wednesday. This along with an increase in warm air advection and rising heights will result in temperatures climbing back above average Wednesday through Friday. Highs for most of the area are forecast to be in the mid/upper 60s Wednesday, then into the 70s for Thursday and Friday. A front may arrive Friday Night into Saturday with some showers, and with the low-level flow more onshore it will bring in some cooling starting Friday especially closer to the coast. If the marine influence ends up being stronger Friday into Saturday, then more low clouds may come into play along with even cooler temperatures.

In terms of rainfall, a weak area of low pressure slides by Wednesday morning bringing some showers. Rainfall amounts generally up to a tenth of an inch are expected. Another area of low pressure may arrive during the weekend, especially for Sunday, bringing the potential for widespread showers or a period of rain. The strength and timing of the involved features for the weekend however is lower confidence. The pattern that is currently indicated among the model guidance however would result in temperatures dropping to average or below average over the weekend, with an increased chance for rainfall.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas.

Tonight...VFR with mostly clear skies. North winds around 4-8 kt becoming light and variable at times. High confidence.

Tuesday...VFR with increasing clouds in the afternoon. North winds in the morning, transition to southwest around 5-10 kt in the afternoon. High confidence.

Outlook...

Tuesday Night...VFR. No significant weather.

Wednesday...MVFR/IFR conditions expected especially in the morning with showers and low clouds around.

Wednesday Night through Friday...VFR. No significant weather.

Friday Night through Saturday...Sub-VFR conditions likely at times with showers and low clouds moving through.

MARINE

No marine headlines are in effect through Tuesday. North-northwest winds around 10-20 kt are expected to continue through tonight before diminishing to 5-10 kt by Tuesday morning. Winds will gradually shift to south-southeast around 10 kt by Tuesday afternoon. Seas around 2-3 feet.

Aside for some rain/graupel showers expected this afternoon, fair weather is expected through Tuesday.

Outlook...

Tuesday Night/Wednesday Morning...Small Craft Advisory conditions possible (30-40%) with wind gusts near 25 kt.

Wednesday Afternoon through Saturday...No marine headlines expected.

CLIMATE

Record low temperatures are possible at some climate sites for Tuesday, April 21st. Below are the daily low temperature records for 4/21:

Daily Record Low Temperatures April 21 Site Record/Year Allentown (ABE) 28/1925 & 1956 & 2018 AC Airport (ACY) 25/1981 AC Marina (55N) 26/1875 Georgetown (GED) 27/1961 Mount Pocono (MPO) 11/1908 Philadelphia (PHL) 27/1875 Reading (RDG) 26/1982 Trenton (TTN) 30/1907 & 1925 Wilmington (ILG) 28/1925

PHI WATCHES/WARNINGS/ADVISORIES

PA...Freeze Warning from midnight tonight to 9 AM EDT Tuesday for PAZ060>062-070-071-101>106. Freeze Warning until 10 AM EDT Tuesday for PAZ054-055. NJ...Freeze Warning from midnight tonight to 9 AM EDT Tuesday for NJZ007>010-012>023-027. Freeze Warning until 10 AM EDT Tuesday for NJZ001. DE...Freeze Warning from midnight tonight to 9 AM EDT Tuesday for DEZ001. Freeze Warning from 2 AM to 9 AM EDT Tuesday for DEZ002-003. MD...Freeze Warning from 2 AM to 9 AM EDT Tuesday for MDZ012-015- 019-020. MARINE...None.