textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes to the forecast were needed.

KEY MESSAGES

1. High pressure brings mostly dry weather and seasonably warm temperatures with comfortable humidity through Sunday.

2. Hot and humid conditions bring Moderate to Major Heat Risk Monday- Wednesday with the peak occuring on Tuesday. Humidity abates the second half of the week, while temperatures continue to run above normal.

3. A cold front brings the threat for severe storms late Tuesday into Tuesday night.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A 500mb shortwave may generate a couple of light showers this afternoon and evening, but otherwise high pressure will continue to build in from the north through tonight and Sunday, keeping conditions dry the rest of this weekend.

This afternoon's cumulus field will fade with sunset, and then mostly clear skies and light winds will allow good radiational cooling tonight. Will continue to go cooler than the NBM and lean more on MOS, which puts lows mostly in the 50s, except 40s in some of the northern valleys. Valley fog is possible in some areas, especially along the CT River.

For Sunday, mostly sunny skies are expected with temperatures forecast to be mainly in the low-mid 80s with dewpoints in the 50s, keeping humidity levels comfortable. A weak pressure gradient and light northerly flow will allow an afternoon seabreeze, so coastal areas may temperatures stay in the 70s. Sunday night is expected to be similar to tonight with temperatures mostly in the 50s (and possible 40s across the north) and patchy valley fog.

KEY MESSAGE 2 DESCRIPTION...

Anomalous ridging responsible for all time record heat in portions of Montana will stretch eastward Monday and Tuesday. As the ridge stretches east temperatures at 850 mb will climb to +15C to +17C Monday and +20C to +23C Tuesday. These temperatures aloft will translate to highs in the low 90s Monday and mid to upper 90s Tuesday. Overnight lows in the upper 60s Monday night and in the 70s Tuesday night will bring cumulative heat stress into Wednesday. Dew points in the 50s and low 60s Monday will keep the humidity in check, while on Tuesday dew points in the mid to upper 60s will bring heat indices 95-100F across much of the area south of the mountains.

A cold front crosses Tuesday night into Wednesday morning. This front will bring in drier air allowing dew points to drop back into the 50s while hot conditions continue with highs into the 90s south of the mountains Wednesday. Troughing gradually deepens over the Northeast through the end of the week that will help high temperatures sink back into the 80s with lows in 50s and 60s.

KEY MESSAGE 3 DESCRIPTION...

A seasonably strong trough will track across SE Canada Tuesday that will send a cold front into the region late Tuesday through Tuesday night. The hot and humid air mass in place combined with steep mid level lapse rates of 7.5C/km will bring MU CAPE greater than 1500 J/kg across the area ahead of the front. The strength of the trough and cold front will bring deep layer shear on the order of 40-50 kts which will promote organized convection. The latest ECMWF EFI shows the combination of CAPE and shear will approach the 99th percentile near the Canadian Border and near the 90th percentile across the mountains. The latest consensus of model solutions suggest the front, and therefore trigger for storms, will not make it to the area until Tuesday evening. However, features such as pre-frontal troughs are poorly resolved at this time range so cannot rule out storms developing ahead of the front. Additionally, strong height falls through the first half of Tuesday night will help compensate for the loss of heating allowing for the threat of severe storms to continue past sunset. SPC maintains a 15 percent probability for severe storms on their Day 4 outlook across much of the area and this is in line with Machine Learning probabilities out of CSU. Will have to monitor timing trends with the front as a faster arrival would lead to more widespread storms Tuesday afternoon. The front will push towards the coast Wednesday morning with the severe storm threat likely diminishing after midnight.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Through 18Z Sunday...IFR to LIFR from valley fog possible at HIE and LEB late tonight into early Sunday morning. Otherwise prevailing VFR expected through the TAF period.

Outlook:

Rest of Sunday: VFR.

Sunday night: IFR to LIFR from valley fog possible at HIE and LEB. Otherwise VFR.

Monday through Thursday: Mainly VFR. Cold front crossing late Tuesday and Tuesday night will bring storms that could bring TEMPO restrictions.

MARINE

Through Sunday night...High pressure continues to build across the waters this weekend, providing tranquil conditions. Light northerly winds tonight and early Sunday will turn more southerly Sunday afternoon as the seabreeze develops. Winds remain south to southwest Sunday night with the high pressure center moves southeast of the waters.

Southwest to south winds will gust 25 to 30 kts during the day Monday and Tuesday into Tuesday night with seas to around 5 feet. A cold front crosses Wednesday morning shifting winds offshore into Thursday.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.