textproduct: Gray - Portland

WHAT HAS CHANGED

Have issued a Small Craft Advisory for gusty southwest winds ahead of the approaching cold front Wednesday into Thursday.

KEY MESSAGES

1. A cold front will slowly approach the region through Wednesday. It will be our best chance for rainfall over the next week, but looks unlikely to provide significant drought relief.

2. Gusty with lower humidity Thursday. Mostly dry weather then looks to continue into early next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The backdoor front has slipped south of most of the forecast area this afternoon, but I am not anticipating too much weather from that feature any longer. It may, very late tonight, provide a focus for some potential showers as WAA kicks in. While CAMs do show some simulated reflectivity over the area, forecast soundings remain quite dry. So I opted to go with sprinkles rather than showers to put some mention of the possibility after midnight. Otherwise it will be cool north of the front, but south of the front where return flow starts overnight readings should remain in the 50s.

Temps look like a similar split across the forecast area Wed. Warm readings south of the boundary and 60s to near 70 north of it. Southwest winds may get gusty, especially late in the day, up to 25 to 30 mph.

Rain showers begin to arrive in the evening and overnight. The diurnal peak of instability is going to line up better for the Finger Lakes into the Hudson Valley, with convection dying as it approaches the Connecticut River. Much like similar weakening cold frontal events this summer, much of the area is looking like a tenth of an inch or less QPF. A few lucky locations may see up to around a half inch. We will take any rainfall, but this does not look like an event that will significant improve drought conditions.

KEY MESSAGE 2 DESCRIPTION... The trend is going to be toward drier weather and clearer skies Thursday as low pressure continues eastward into the Gulf of St. Lawrence sweeping the front through. The front will drop the dewpoints back to comfortable levels, but continued southwesterly flow maintains 850 mb temperatures +12-15 C south of the mountains. Mixing profiles from forecast soundings look good, so assuming we do clear out fairly quickly high temperatures in the low to mid-80s are not out of the question. North of the mountains cooler air is going to be filling in behind the front and capping temperatures in the low to mid-70s. A 50kt low level jet also sets up over the region with 850 mb winds more in the 25-30 knot range. As it stands now most guidance suggests mixing up to 850 mb looks likely, but unsurprisingly only the NAM is convinced of higher potential. That being said I heavily tailored the forecast toward the 90th percentile of the NBM to get the 30-35 mph gusts in there that would be expected if we are mixing down 25-30 knot winds. A much cooler night is expected as winds go calm and skies trend mostly clear. North of the mountains could see temperatures fall into the low to mid-40s, with upper 40s to low 50s in the foot hills and the coastal plain and southern New Hampshire ending up in the mid to upper 50s.

Friday may also be a fairly breezy day with gradient wind gusts 20- 25 mph as high pressure begins to build into the area and low pressure continues to slowly move eastward to Atlantic Canada. Northwesterly flow will make for a seasonably cool day north of the mountains as temperatures likely max out in the 60s. South of the mountains should see a more seasonable day with temperatures in the low to mid 70s. Friday night looks like a good candidate for radiational cooling as skies remain clear and winds calm. I wouldn't be surprised if low temperatures end up in the low to mid 40s across most of the area. High pressure than slides overhead Saturday bringing calmer conditions. This is likely to be the pick of the week as the upper level ridging should allow for high temperatures in the 70s across the board under clear skies. The high pressure center moves off the coast Saturday night and the resulting southwesterly flow keeps low temperatures in the upper 40s and 50s.

Sunday into early next week are more unsettled as global models want to bring another front through in the Sunday PM/Monday window, but global ensembles don't have a very strong signal for widespread precipitation and with how things have gone this summer I don't want to speculate. Therefore, I am fine with the NBM advertising slight chance showers south of the mountains with chance showers in the north (aided by the northwesterly flow).

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through Wednesday...Widespread VFR conditions expected thru Wed. Some local IFR or lower is possible in the Connecticut River Valley tonight, including LEB and HIE, but southwest winds may disrupt or delay that. Showers move into the area after sunset Wed. Local MVFR or lower is possible in any shower, but fairly quick improvement to VFR is expected. This will be especially true once winds turn northwesterly early Thu.

Outlook:

Thursday: Mainly VFR with west winds gusting 25-30 kt, possibly as high as 35 kt.

Thursday night-Saturday: Mainly VFR. Valley fog possible at HIE and LEB by Friday night.

Sunday-Sunday night: Generally VFR, with MVFR possible as showers move into the area.

MARINE

Winds and seas are expected to remain below SCA thresholds thru Wed afternoon. Then southwest winds increase Wed night. While it will remain fairly stable, low level winds look strong enough for widespread gusts at or above 25 kt and seas building above 5 ft outside of the bays. I have gone ahead an issued a SCA to cover this period into Thu.

SCA conditions are likely for at least a portion of our coastal waters through the day Thursday, but the trend will be downward so by Thursday night wave heights and wind gusts should fall below SCA criteria. High pressure than builds over the Northeast, eventually sliding over the waters through the weekend. Generally, sub-SCA conditions look to continue into early next week as high pressure largely remains in control over the waters.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory from 8 PM Wednesday to 8 PM EDT Thursday for ANZ150-152-154. Small Craft Advisory from 8 PM Wednesday to 8 AM EDT Thursday for ANZ151-153.