textproduct: Burlington

WHAT HAS CHANGED

As of 305 PM EDT Saturday...

Greater confidence in sharply colder daytime temperatures on Tuesday, especially in eastern Vermont and St. Lawrence Valley.

KEY MESSAGES

As of 305 PM EDT Saturday...

1. Rain associated with the remnants of Isaias will bring a soaking rainfall Sunday night with no significant impacts for our area.

2. An extended wet period from much of Monday night through Wednesday morning is expected, associated with a slow moving and thermally strong cold front.

3. Seasonably cool conditions in the middle to end of the week, with a couple shower chances Wednesday, and a more defined system potentially late week into the weekend.

DISCUSSION

As of 305 PM EDT Saturday...

KEY MESSAGE 1:

As seasonably strong high pressure (~1030 millibars) moves to our southeast tonight, a tightening pressure gradient will develop with modest south/southeast flow beneath thickening high altitude clouds. Most notably terrain effects will lead to another breezy period tomorrow morning on the east side of Lake Champlain in the northern portions of the valley, where gusts should exceed 30 MPH for much of the day and a Lake Wind Advisory will probably be needed. Otherwise, we will await rain overspreading the region from the south as the remnants of post-tropical cyclone Isaias move northeastward near the Appalachian Mountains. The low track will then be shunted to the east across the Mid-Atlantic region Sunday night, but far enough north such that plenty of lift and moisture will reach our latitude before precipitation gradually exits to our east overnight.

High confidence of some rain, starting roughly sunset in southern portions of the region and towards midnight in northeastern Vermont, remains on track. Biggest impact of this rain would probably be related to hydroplaning following an extended dry period, but overall it should be a beneficial rainfall. Most guidance shows a relatively short duration of lighter rain towards the International Border where PoPs are somewhat lower on an hourly basis, but think at least some rain will occur. Generally greater rainfall amounts are more likely as you go southward with this event given the storm track. However, strong mid-level frontogenesis will help produce strips of heavier rainfall oriented from west to east, which are always difficult to pin down. Rainfall rates for at least a few hours will increase within these bands, likely 0.1-0.2" an hour, well shy of truly heavy rain. As such, localized pockets of heavier rain can be expected; think the 90th percentile amounts in the 1 to 1.5" range look reasonable as a reasonable high end amount which still would not produce any hazardous hydrological conditions. That said, the deterministic forecast is fairly close to the REFS probability matched mean with more modest amounts; most likely storm totals in our northernmost counties are less than 0.1" while reaching about 0.5"-0.75" in Rutland and Windsor counties; most areas in between will vary widely between 0.1"-0.5". Most likely end time of rain in most locations is during the Monday morning hours roughly from 2 to 5 AM in northernmost Vermont and northern New York and 5 to 8 AM in the rest of Vermont. A very narrow ridge will follow ahead our next weather system providing mainly dry conditions during the day Monday, aside from localized showers/drizzle in the mountains with lingering moisture possibly trapped below an inversion around 2 to 3 thousand feet elevation. Valley locations will probably see their warmest day of the week as a result.

KEY MESSAGE 2: Following the rain on Sunday night, another high probability rainfall event (about 100% chance of rain) continues to be expected for Tuesday. Moisture from the remnants of Tropical Storm Rachel in the eastern Pacific will be carried out ahead of a strengthening front approaching from the north while high pressure builds in across eastern Canada, producing a cold air damming setup for New England.

Rainfall amounts could be rather substantial, with high statistical confidence of totals from Tuesday through Tuesday night exceeding 0.5" for nearly all of our region, and even 1" probabilities are in the 50-70% range in much of northern New York and northern portions of Vermont. While the setup for widespread rain with this boundary stalling out while a wave of low pressure passing through our region is largely in good agreement, the position of this system is highly uncertain.

Even with a relatively short lead time (day 4), there is very large spread in the upper air pattern and associated position of boundaries that lead to important differences in our sensible weather. The GFS and most of its ensemble members have lower heights upstream, leading to a low pressure track farther north and warmer/more unstable air moving through our region. Meanwhile, the ECMWF and many of its ensemble members, RRFS, and NAM12 show a pronounced cold air damming scenario as shallow cold air gets locked in across the St. Lawrence Valley and eastern/northeastern Vermont. This scenario, in which low pressure would be delayed and pass overhead or even south of our region, would lead to an all day rain event. The aforementioned areas would be quite chilly with temperatures holding in the mid 40s to near 50, while south/southeast winds in the Champlain Valley and Adirondacks transport deep moisture and maintain somewhat milder air. We have trended temperatures this way given the consensus; note there is some merit in the GFS depiction, so the forecast remains unusually uncertain and we can expect non-diurnal temperature fluctuations.

KEY MESSAGE 3: As our Tuesday system departs, decent caa will filter seasonably cool air into the region from the northwest. Wrap around moisture Wednesday associated with cooling aloft, will likely lead to some shallow instability showers across the region. Isolated to scattered showers will be possible, especially in the Northeast Kingdom and Adirondacks. Expectations will likely follow breaks of sun followed by a quick shower. PoPs will likely see increases in the coming days as we get closer to mid week. Winds will also be seasonably breezy with some channeled winds anticipated Wednesday. Brisk breezy northwest winds can be expected. Temperatures will drop slightly Wednesday into Thursday as weak ridging builds into the region. Highs on Wednesday and Thursday will be in the low to mid 50s, with possible near 60s in southern Vermont depending on cloud coverage. Lows Wednesday and Thursday night will dip into the low to mid 30s areawide with some upper 20s across the Adirondacks.

The pattern then becomes quite varied amongst the global models. There is good agreement in some sunshine Thursday, however, the arrival of our next system remains in question. Model timing and handling of a closed low in the Intermountain West is leading to start difference between the GFS and Euro ensembles for precipitation chances late next week into the weekend. Regardless, there does look to be good chances for rain this weekend, but the timing of the system is still uncertain. Additionally, regardless of the timing, as our late week system arrives, a strong 50-60 knot jet looks to be tied to the system which will bring chances for gusty winds, especially for the Champlain and St. Lawrence Valley. GEFS and Euro ensemble probabilities of gusts exceeding 25 MPH for most locations is between 40-60% Friday into Saturday with probabilities of gusts exceeding 35 MPH between 10-30% for the St. Lawrence and northern Champlain Valleys.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

Through 00Z Monday...VFR conditions are expected through 00Z Monday. Tonight will feature light terrain driven winds. Winds will increase from the south after 12Z, especially in the Champlain Valley. Some afternoon gusts 15-25 knots are expected at KBTV and KPBG.

High clouds this evening are expected to thicken and lower tonight through Sunday, though ceilings will remain at VFR levels.

Outlook...

Sunday Night: Mainly VFR, with local IFR possible. Chance RA. Columbus Day: Mainly MVFR, with local VFR possible. NO SIG WX. Monday Night: Mainly MVFR, with local IFR possible. Chance SHRA. Tuesday: Mainly MVFR, with local IFR possible. Definite SHRA, Slight chance TSRA. Tuesday Night: MVFR. Definite SHRA, Slight chance TSRA. Wednesday: Mainly MVFR, with local IFR possible. Slight chance SHRA. Wednesday Night: Mainly MVFR, with local VFR possible. Slight chance SHRA. Thursday: VFR. NO SIG WX.

BTV WATCHES/WARNINGS/ADVISORIES

VT...None. NY...None.