textproduct: Gray - Portland

WHAT HAS CHANGED

Last night's downward and southeastward trend in heaviest rainfall has continued with today's guidance.

KEY MESSAGES

1. Low pressure brings soaking rainfall through Tuesday morning. Rainfall amounts are forecast to remain below flash flood guidance, while embedded heavy rain could bring some urban and poor drainage issues along the coast and southern NH.

2. Temperatures rebound Wednesday and Thursday, but weak disturbances keep showers possible. Low pressure moves across the forecast area Friday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The reversal from yesterday's 12Z guidance jump in QPF has continued with today's 12Z guidance. Latest surface analysis places an elongated area of low pressure across the Ohio Valley associated with a couple of low amplitude waves within broad cyclonic flow aloft. The leading wave will cross the area tonight while the trailing wave exits off the Mid Atlantic Tuesday afternoon. The disconnect between these waves will result in weaker waves of low pressure tracking near southern New England through Tuesday rather than a more organized area of low pressure tracking closer to the forecast area. As such, mean QPF from today's 12Z HREF has dropped and shifted south compared to yesterday's 12Z run. However, the HREF max continues to show potential for a narrow axis of rainfall of 3-4 inches near the coast and southern NH. Breaking it down to the HREF 3-hour max amounts suggest rates will likely remain below 3-hour flash flood guidance. Will have to watch the potential for urban poor drainage issues tonight into Tuesday morning across southern NH and coastal Maine. Yesterday, portions of central Maine towards the Mid Coast did receive heavy rainfall and this area will be at the greatest risk of seeing the renewed threat of flash flooding as CAMs have been highlighting the Camden Hills area for heavy rainfall.

The going QPF forecast brings 1-2 inches along and south of the foothills with the potential for localized amounts to around 3 inches. Latest radar has stratiform rain pressing into SW NH and this will spread northeastward through this evening. Some instability ahead of this stratiform rain that will allow for some isolated thunder showers. CAMs are in decent agreement that a plume of MU CAPE around 250 J/kg will press into southern NH and coastal Maine allowing for some embedded thunder overnight. It is this instability combined with PWATs nearing 1.75 inches that will allow for heavier rainfall rates while the HREF probabilities for 1 inch per hour rates are very low. The bulk of the rainfall will come to an end Tuesday morning while the wave exiting the Mid Atlantic will keep chances for showers going across southern NH and near the coast into the afternoon before a clearing trend into the evening. The new rainfall combined with some clearing skies and light winds will likely lead to patchy fog Tuesday night.

KEY MESSAGE 2 DESCRIPTION...

A relatively zonal mid-level pattern is likely through the end of this week. This will allow for the potential to see routine bouts of unsettled weather by Thursday and Friday across northern New England.

Through the second half of the week, instability over the area is likely to generate scattered showers and thunderstorms. Temperatures also look a little warmer, with high temperatures in the upper 70s and lower 80s through the rest of the week. Wednesday has the best chance to be the driest day of the three, with scattered thunderstorms more likely for Thursday.

Heading into the weekend, a surface low pressure moves in from the Great Lakes. This low will follow a mid-level trough and could allow for the potential to see widespread soaking rain through much of the day on Friday. The aforementioned low moves northeastward by Saturday morning with a drier Saturday possible before more unsettled weather comes into view for Sunday.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Through Tuesday...Clouds thicken and lower from SW to NE with rain overspreading the area. Conditions will lower to MVFR this evening and likely IFR/LIFR overnight for all sites. Rain ends away from the coast and souther NH with cigs starting to lift and scatter late morning. All sites likely improve to VFR Tuesday afternoon.

Outlook...

Tuesday Night: VFR conditions expected. Patchy valley fog possible with local IFR or lower.

Wednesday: VFR Expected.

Thursday: VFR Expected.

Thursday night: MVFR likely due to showers and thunderstorms.

Friday: MVFR likely due to rain and thunderstorms.

Saturday: MVFR possible.

MARINE

Waves of low pressure will track across southern New England through Tuesday. Southeast winds increase overnight with gusts around 20 kts approaching 25 kts along the outer waters. Seas will also build and will generally be 3-4 feet, approaching 5 feet along the outer waters. Light offshore winds develop Tuesday afternoon low pressure exits. Winds and seas remain below SCA thresholds through the end of the week while a cold front approaches late in the week.

GYX WATCHES/WARNINGS/ADVISORIES

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