textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. High pressure sinks south Sunday followed by a cold front bringing chances for showers and thunderstorms late Sunday into Sunday night. High pressure returns Monday.

2. Multiple chances for rain next week; greatest likelihood on Tuesday and Wednesday PM-Thursday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

High pressure centered over the Mid Atlantic will drift east tonight through Sunday morning. Mostly fair weather is expected through Sunday morning before a cold front approaches the International Border Sunday afternoon. This front will bring chances for showers and thunderstorms Sunday afternoon into the evening across the mountains into portions of central Maine. A dry airmass in place will limit instability with MU CAPE generally less than 1000 J/kg. Therefore, the threat for severe storms is low while deep inverted V low level profiles may allow for gusty outflow winds with any stronger cells. Areas south of the mountains and southwest of the Capital District of Maine likely remain dry into sunset. Southwest winds will increase through the morning ahead of the front with gusts 15 to 20 mph by early afternoon. These winds will allow for mixing to around 850 mb where temperatures will be close to +15C leading to highs south of the mountains into the mid 80s. North of the mountains highs will be in the 70s.

The cold front will continue to press southward Sunday night with shower chances expanding south towards the coast the first half of Sunday night. Shower chances diminish the second half of the night as the front pushes offshore. High pressure then builds in behind the front for fair weather on Monday with highs in the 70s to low 80s.

KEY MESSAGE 2 DESCRIPTION...

Fast WNW 500 mb flow overspreads the Northeast U.S., on the periphery of a strong ridge to our south. It will be a favorable setup for disturbances to ride the ridge and bring some rain chances to New England. Guidance has been all over the place with the timing of these waves, but beginning to see agreement in Tuesday, then again on Wednesday PM-Thursday, as the two windows of opportunity for more widespread rainfall. Some drier stretches are expected outside of these two windows, but really cannot rule out a shower from popping up in this ring of fire type pattern at any time through next week.

There are certainly signals for multiple rounds of heavy rainfall in this pattern, but the spatial extent and whether it is centered over our forecast area remain uncertain. Ensemble guidance currently shows a near 50% chance of exceeding 1" QPF for Tuesday-Friday, while that number drops to 10-20% for >2". In summary, there is certainly the potential for some much needed rainfall, but generally not concerned about excessive rainfall at this time, likely owing to a rather progressive nature to the flow pattern.

Temperatures are also lower confidence primarily because of a warm front/quasi-stationary front that sets up in the vicinity. ECMWF/NAM brings the warm sector north into SW NH on Tuesday, while other guidance keeps it well south of the forecast area. Am skeptical on a northern outcome especially since morning rain/convection looks to be involved. Shifting to Wednesday-Thursday, all guidance keeps us north of the front. This all is likely to result in cooler highs in the 60s-70s, but mild/muggy nights with dew points in the upper 50s/low 60s the floor for low temperatures.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Through Sunday night...VFR likely prevails through Sunday except for valley fog tonight likely bringing restrictions to HIE and LEB. SHRA/TSRA develop near the Canadian Border Sunday afternoon and spread southward through 06Z Monday. Low cigs and SHRA/TSRA may bring restrictions to HIE and AUG late Sunday afternoon into Sunday night with a trend towards VFR Monday morning.

Outlook...

Monday night: Mainly VFR, no sig wx.

Tuesday-Thursday: Lowering cigs and SHRA likely bring periods of at least MVFR.

MARINE

Winds and seas remain below SCA thresholds through the middle of next week. High pressure over the Mid Atlantic drifts east tonight and settles south of the waters Sunday. SW winds increase into Sunday afternoon with gusts to 20 kts and seas around 3 feet. A cold front crosses Sunday night shifting winds offshore followed by high pressure on Monday. A front drifts north towards the waters through mid week with prevailing SE to E flow and chances for showers.

GYX WATCHES/WARNINGS/ADVISORIES

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