textproduct: Boston / Norton

WHAT HAS CHANGED

A quick burst of snow may impact parts of the morning commute Wednesday. Latest guidance has trended the system Thursday/Friday further south and away from the region, but we will continue to monitor this.

KEY MESSAGES

- A quick burst of snow very late tonight and Wednesday morning will bring 1-3 inches accumulation, then turning milder in the afternoon.

- Weaker coastal low pressure may bring some unsettled weather sometime late Thursday into Friday, but has been trending south away from the region.

- Drier for the weekend before the risk for more unsettled weather returns to start the week

DISCUSSION

Key Message 1...A quick burst of snow very late tonight and Wednesday morning will 1-3 inches accumulation, then turning milder in the afternoon.

Brief shortwave ridging this evening will bring a period of mostly clear skies with some cirrus which will allow for decent radiational cooling during the first half of the night. Temps expected to drop into the single numbers and teens before rising late tonight as southerly winds develop and clouds thicken ahead of next system. A mid level shortwave moves eastward from the Gt Lakes late tonight into Wed as a warm front approaches from the SW. Decent shot of warm advection/isentropic lift develops which will be enhanced at the nose of a 40 kt low level jet. This will result in snow developing from west to east late tonight and Wed morning. This will be a minor QPF event with snowfall averaging 1-3 inches but much of it could fall in a 3 hour period.

While the higher amounts are favored over the higher elevations, it is possible there could be a secondary max just inland from the south coast across RI and SE MA. Interesting set-up with low level jet overriding an antecedent cold airmass. Given the low level cold air initially in place, the stronger southerly winds may take a bit longer to mix down to the surface near the coast which would be enough for a quick burst of snow with brief 1"/hr rates possible as suggested by the HREF. This would be from the enhanced forcing for ascent at the nose of the LLJ. If the southerly winds were quicker to mix to the surface, this would diminish the risk for this brief burst of snow near the south coast. Eventually the south winds will increase and temps will rise above freezing with snow probably ending as some rain near the coast. This is a quick moving system and snow will be ending from W to E around midday or early afternoon but the core of the event will probably be in a 3 hr period. The snow is expected to develop from 5-8 AM from W to E so it will impact parts of the morning commute, especially western New Eng where the snow begins earlier. After the snow ends, temps will rise through the 30s with highs upper 30s, except holding in the low-mid 30s higher elevations. Gusty S-SW winds will develop near the coast Wed morning with gusts to 25-30 mph as the low level jet moves across the region.

Key Message 2...Weaker coastal low pressure may bring some unsettled weather sometime late Thursday into Friday, but has been trending south away from the region.

Another coastal low exiting offshore of the Carolina coast may pass south of the region some time late Thursday into Friday. The latest guidance has trended the system further south and away from the region, but a grazing pass along the south coast and Islands is not out of the question just yet. Ensemble probs have trended down for measurable snow Friday morning, ranging between 20-40% over the south coast across the ensemble suites. The GEFS and ECMWF ENS are the more bullish ensembles on this, with the GEPS only hinting at an area around the Cape with 30% probs for measurable snow Friday morning. Will need to continue to monitor, but the latest trends are favoring lesser impacts if any at all with this system.

Key Message 3...Drier for the weekend before the risk for more unsettled weather returns to start the week

High pressure builds in briefly before going offshore as the day goes on Saturday and a front moves through the region over the weekend, which will end the brief period of possible warmer temperatures Saturday. Temperatures at 925 mb and 850 mb get just above 0C for Saturday afternoon in the midst of more southwesterly flow, which may support high temperatures climbing well into the 40s. A front moves through sometime in the second half of the weekend, injecting another cold shot of air into the region to start the week. Some unsettled weather could return for the start of the week which we will continue to monitor.

AVIATION /19Z TUESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

Tonight and Wednesday...Moderate confidence.

VFR through much of tonight, then snow overspreads the region from W to E from 10-13z with lowering conditions to MVFR-IFR. Snow likely mixing with or changing to rain along immediate south coast and especially Cape/Islands. Snow ends by 18z with improving vsbys but MVFR cigs persist. Increasing S-SW winds develop Wed morning with gusts to 25 kt, up to 30 kt on the Cape/Islands where area of LLWS expected as low level jet moves across the region. Winds and LLWS diminish late Wed as the LLJ moves to the east.

Wednesday night...Moderate confidence.

Areas of MVFR improving to VFR. SW-W wind 5-15 kt, some higher gusts early on the Cape/Islands.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday through Sunday/...

Thursday: VFR. Breezy. Slight chance SN.

Thursday Night through Friday: VFR. Slight chance SN.

Friday Night: VFR.

Saturday: VFR. Breezy.

Saturday Night through Sunday: VFR.

MARINE

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

Small Craft Advisories have been issued for all waters late tonight into Wed for a period of SW gusts to 25-30 kt as a low level jet moves across the waters. Winds will diminish Wed evening but 5-8 ft seas will linger over southern waters through Wed night. Vsbys lower late tonight into Wed morning in a period of snow and rain.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Local rough seas. Slight chance of snow.

Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of snow.

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain, slight chance of snow.

Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft.

Saturday through Sunday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 7 AM to 4 PM EST Wednesday for ANZ230. Small Craft Advisory from 7 AM to 7 PM EST Wednesday for ANZ231-236-251. Small Craft Advisory from 4 AM to 10 PM EST Wednesday for ANZ232>235-237. Small Craft Advisory from 7 AM to 10 PM EST Wednesday for ANZ250. Small Craft Advisory from 4 AM Wednesday to 7 AM EST Thursday for ANZ254>256.