Area Forecast Discussion
Issued by NWS Portland, ME

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265 FXUS61 KGYX 211134 AAA AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 634 AM EST Tue Nov 21 2017 .SYNOPSIS... A warm southwest flow develops today ahead of an approaching cold front. The cold front will move across the region Wednesday followed by another colder air mass. Wednesday scattered rain or snow showers changing to snow showers are expected in the mountains and rain showers elsewhere. Cold high pressure returns for Thursday and Friday with fair and cold weather. High pressure moves off the coast Saturday allowing a warmer return flow to develop ahead of another approaching cold front that will arrive on Saturday night. An upper trough of low pressure settles over the region Sunday with unsettled weather and cold temperatures. && .NEAR TERM /THROUGH TODAY/... Update...Adjusted temps down a few degrees in Srn NH...where clear skies and light winds have allowed temps to bottom out in the 20s to near freezing. Otherwise...no major changes. Previous discussion...A warm front will lift thru the forecast area this morning. Based on cloud cover...the mid level front is more or less halfway thru the forecast area at this time. That is making the hourly temp trends a bit of a bear...as clear skies and light winds on either side of the cloud band are cooling...but beneath the band temps are rising slightly. In general most locations have reached their low temps for the night...except for Srn NH where clouds early in the night kept readings on the milder side. Readings there are now falling as skies clear out behind the front. SW flow aloft will help bring in warmer temps. Readings are expected to climb thru the 40s for most areas...and settle into the low to mid 50s. Slightly cooler readings are expected in Nrn NH...and closer to 40 thru the Wrn ME mtns. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY/... Quiet night on tap as a cold front approaches New England from the W. In SW flow temps will slowly settle towards freezing...especially in the sheltered Nrn valleys. Precip along the front will begin to encroach on Wrn NH around daybreak. Temps...especially across Nrn zones...may be initially cold enough for snow showers. If surface cold front can arrive fast enough...precip may go straight over to snow N of the notches. Regardless...change over should occur pretty quickly in the N. It looks like whatever low level inversion that develops overnight will be erased fairly quickly as the front arrives. For that reason I expect ptypes to be driven by surface temps. That is if there is even widespread precip from the cold front S of the mtns. What is more likely to produce rainfall is moisture streaming Nwd along the coast ahead of the front. Sly flow around the periphery of Atlantic high pressure and Wly flow developing inland will produce a nice area of sloped frontogenesis from Long Island thru Downeast ME. With deep enough moisture this is enough forcing to warrant likely PoP along the coast. This frontogenesis develops close to 12z...and lifts NEwd by early afternoon as the front crosses the rest of the forecast area. Behind the front upslope snow showers will linger in the mtns...though forecast Froude numbers near 1 look to keep the bulk of precip near and NW of the peaks. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... The outlook period will be dominated by a general carving out of a broad long wave trof position in the east with ridging across the west central US. A fast flow aloft will allow several short wave trofs to move through this flow which will mean several fast moving changes in air masses over the next 5 days with a much colder air mass to become more entrenched by later in the weekend as the upper trof becomes more amplified over the northeastern US. Breaking it down - High pressure settles over the region Thursday and Friday with a return to colder temperatures and dry conditions. By Sat the low and mid level flow briefly becomes southwest allowing a warmer transport of air. This will be brief but allow for a mild day Sat as another trof approaches from the west with an associated cold front. That cold front moves through the region Sat night producing scattered showers changing to snow showers in the mountains with just scattered rain showers elsewhere. Sunday the models are suggesting an amplifying upper trof over New England which would provide some unsettled conditions and combined with strong cold air advection, most of the PTYPE would be in the frozen category. Considering the system will be moisture starved only light amounts of QPF are expected with the upper trof on Sunday. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... Short Term...Some scattered MVFR CIGs possible this morning...especially in the mtns...as a warm front lifts across the forecast area. Light surface winds and stronger Wly flow near the Canadian border will allow for some marginal LLWS conditions at HIE through today. VFR conditions expected today at all terminals...with gusty SW flow developing...though gusts should remain at or below 20 kts. Increasing SW flow aloft may bring more widespread LLWS conditions to the forecast area...though confidence was too low to include in the most recent set of TAFs. 12z sounding will help inform that decision...as it will provide a gauge as to how model guidance is handling low level wind speeds. Cold front approaching Wed...and will spread showers across the area. Some SHSN are possible in the mtns...but I expect all terminals to see SHRA during the first part of the day. Best chance for MVFR conditions will be near the coast...and NW of the mtns. Colder air filtering into the region in the afternoon will allow the threat for some upslope SHSN at HIE and local IFR conditions. Long Term...Mainly VFR conditions through Sat. && .MARINE... Short Term...Yet another lull in the winds this morning...as a warm front lifts across the waters. SW flow will increase thru the day...and SCA conditions are expected to develop mid morning to early afternoon. Winds decrease in the bays this evening...and late tonight outside the bays...though seas may linger above 5 ft for a time. I have extended the SCA for the outer waters...and issued a separate SCA for the bays. Ahead of the approaching cold front Wed...moisture will be drawn in towards the coast. Some light rain is likely for the waters during the first part of the day. CAA and gusty NW flow picks up behind the front for Wed afternoon. Long Term...NW flow with gusty winds may provide SCA conditions Wed night. Thu through Fri high pres settles over the waters with light winds. Sat an increasing SW flow ahead of another approaching cold front may require a SCA. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory from 10 AM this morning to 10 PM EST this evening for ANZ151-153. Small Craft Advisory until 5 AM EST Wednesday for ANZ150-152- 154. && $$ NEAR TERM...Legro SHORT TERM...Legro LONG TERM...Marine

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