Scientific Forecaster Discussion
fxus61 kgyx 081406
Area forecast discussion
National Weather Service Gray ME
906 am EST sun Dec 8 2019
high pressure shifts offshore today allowing a warm and moist
return flow to develop tonight. By Monday, temperatures in the
40s will be common. But along with the milder weather will come
rain as a low pressure system crosses the region Monday and
Tuesday. Much colder air follows this system for the remainder
of the week with an active weather pattern to continue.
Near term /until 6 PM this evening/...
905 am...at 13z a 1036 millibar high was centered over southern
New England. GOES visible imagery showed clear skies across the
region at moment under the cresting surface high. Sunshine was
allowing the Mercury to quickly rebound from frigid early morning
readings. For this estf update...I ingested the latest mesonet
into near term grids.
Prev disc... update... have updated the
forecast based on current observations and latest mesoscale
models. Record breaking cold starts the day under clear skies
this morning. Much of the area saw lows below zero last night
with light winds and radiational cooling.
Temperatures will rebound to near freezing over southern areas
today with plenty of sunshine this morning. Clouds will increase
however as the day GOES on as the next system approaches from
warm air advection will begin today after a very
cold morning. The atmosphere will be relatively dry at the
onset, however there could be additional snow showers in the
higher terrain over northern areas.
With some sunshine for the first half of the day temperatures
will rebound, but still remain below normal for mid December.
Highs will reach the 20s in the north to around the freezing
mark in the south.
Short term /6 PM this evening through Monday night/...
a broad and moistening southwesterly gradient will continue
tonight with light snow possible in the warm air advection
pattern over northern areas. A period of gusty winds is possible
tonight as the pressure gradient increases between the surface
ridge exiting off the coastline and an approaching frontal
system to our west. Much of this wind however may skip over the
surface with windy conditions likely over the elevated terrain.
Temperatures will increase during the night in a non-diurnal
Low pressure will gradually intensify and move towards the Great
Lakes region on Monday in association with a large upper level
trough. Warm air and higher dew points will continue to enter
the region on southwesterly winds throughout a deep layer. Rain
will spread through southern areas and with time the
precipitation will increase over northern areas mainly in the
form of snow. In-between, there may be a period of sleet or
possibly a few towns with light freezing rain. However, the
predominant ptype will be rain or snow going to rain.
As dew points increase over the snowpack, fog will begin to
become more widespread as well during the day.
00z model suite in good agreement continuing to intensify an
area of low pressure as it moves through Quebec province Monday
night. Rain and fog will continue. The pressure gradient will
allow for very gusty winds as well, especially along the
midcoast region which will be exposed to the south and southwest
off the Gulf of Maine. The precipitation will allow for
additional snowmelt, leading to rises on small river and streams
and the potential for ponding on roadways and culverts.
Long term /Tuesday through Saturday/...
Tuesday, dry air intrudes aloft behind a mid-level short wave
trough axis, but still ahead of the broad, long wave upper-level
trough centered over Hudson Bay. This along with a lull in
forcing leads to a lull in precipitation during the day with
scattered shower activity and generally light qpf, at best.
Later on, a trailing cold front crosses west to east, helped
along by a closed mid- level low rotating NE Ward around the
front side of the parent upper trough as the latter digs deeper
into on/qc. Surface fropa, crossing into the Gulf of Maine
Tuesday evening, will enhance precipitation chances somewhat as
cold advection through the column switches remaining rain to
snow by Tuesday night before drying out. Favorable upslope flow
develops in the wake of The Crossing cold front as well, so snow
showers are expected Tuesday evening and night across the
mountains as long as low level moisture remains.
Models have been fairly consistent with the timing of frontal passage on
Tuesday, noting that the European model (ecmwf) has trended toward the GFS
solution recently. Thus, confidence is increasing that
temperatures warm well above average, into the 40s and 50s,
Tuesday in the warm sector. Peaks of sun may allow temperatures
to make a run at 60, perhaps warmer, in some spots as h850
temperatures along the coastal plain exceed 6c. Behind the
front, a sharp airmass change is expected as h850 temperatures
plunge well below freezing, allowing surface temperatures to dip
into the teens and 20s (near 30 along southern coastal areas)
Wednesday morning. Ensemble analysis suggests some uncertainty
in the depth of the mid-level low, so questions remain in the
strength of forcing, namely how much "oomph" will the front
have before slowing over the Atlantic early Wednesday. On a
larger scale, models start to diverge mid- week on the extent to
which the long- wave trough digs into the Great Lakes region
and thus the strength of pgf aloft and related dynamic forcing.
Wednesday, forcing returns in response to a strengthening jet aloft,
on the front side of the digging long wave trough. In spite of
uncertainty in strength, model agreement does exist on h300 winds
increasing to well over 150 kts over eastern Continental U.S./Canada which
supports the development of yet another shortwave over the
northeast. This may bring in one final round of precipitation into
the area sometime Wednesday with around 0.25" additional quantitative precipitation forecast
possible in the form of snow or a rain/snow mix. Whether or not this
final round of precipitation falls over land will rely on the
position of the stalled surface boundary over the Atlantic, thus
highest pops will be along the coast, capped at high chance, waning
late in the day. As has been noted in internal collaboration with
wpc, may need to keep an eye on the Wednesday time period for
accumulating snowfall potential with strong dynamics, a cool
airmass in place, and uncertainty in precip placement.
For the back half of the week, the long wave trough continues to
nudge east into northeast noam, continuing to entrench the area
in a cool airmass. A small clipper system driven by the mean
trough crosses sometime late Wednesday into Thursday before
strong high pressure moves in Thursday night. Took temperatures
down Friday morning as a result, into the single digits and
teens, with a caveat of high clouds possibly capping
radiational cooling. This weekend, temperatures moderate closer
to normal in response to another low pressure system approaching
from the southwest.
Aviation /14z Sunday through Thursday/...
short term...a few flurries will continue in the mountains this
morning. Otherwise, VFR conditions. Ceilings begin to lower
tonight, mainly over northern areas with some snow shower
activity. Areas of rain and snow enter the region from southwest
to northeast Monday morning with conditions lowering to IFR and
LIFR in some locations with the development of fog over the
eroding snowpack. IFR/LIFR conditions continue Monday night.
Long term...IFR restrictions likely lift for a short time
Tuesday, coinciding with a lull in precipitation amid breezy
southwest flow on the order of 10-15kts. Later in the day, a
cold front crosses west to east with rain switching to snow and
a westerly to northwesterly wind shift in its wake Tuesday
night. MVFR restrictions would be expected in -ra, but -sn would
introduce IFR or worse visibilities. Another possible wave of
snow would lead to widespread restrictions Wednesday however there
remains uncertainty in the position of this wave with best odds
over the coastal plain. High pressure builds in Thu/Fri with VFR
short term...challenging forecast for the coastal waters this
morning. Winds begin to back out of the southwest today as high
pressure settles off the eastern Seaboard. The southwest
strengthens tonight in response to the pressure gradient between
the exiting high pressure system and an approaching front to the
west. It appears all areas except Casco Bay may have a brief
period of gale force winds tonight with this feature. Have
therefore issued gale warnings for areas outside Casco Bay.
On Monday, there will be a lull in the wind field as the high
continues to move offshore. Winds drop below gales during the
day, only to increase once again Monday night as an intensifying low
area of pressure passes well to the northwest of the waters.
Gale force conditions can be expected once again, this time in
Casco Bay as well.
The complex wind fields warrant the gale warnings which will be
in effect tonight through Monday night. Again, there will be a
lull on Monday, but for simplicity and messaging have kept one
Gale Warning in place. Wind fields will need to be monitored for
the small possibility of the gradient allowing for a brief
period of storm force winds Monday night.
Long term...Small Craft Advisory seas and winds continue Tuesday with gusty
northwest winds tapering off Tuesday night. Periods of rain, Small Craft Advisory
seas, and Small Craft Advisory winds remain possible through early Thursday,
then high pressure moves overhead and allows for a relatively
tranquil day or two of weather on the waters.
mild temperatures, high dew points, and periods of rain will
impact the area Monday and Tuesday and erode snow cover across
the region, particularly over southern New Hampshire and
coastal western Maine. While the fresh snow will initially
absorb rainfall, the snow will likely reach capacity and begin
melting. This will lead to an influx of water into local
waterways. At this time, southern New Hampshire has over a foot
of snow in places with liquid equivalent likely exceeding 1",
with some areas possibly closer to 2". Should moderate rainfall
occur in areas with high snow water equivalent, instances of
localized flooding will be possible in poor drainage areas, as
well as some main Stem rivers and tributaries rising above
Marine...Gale Warning from 6 PM Monday to 5 am EST Tuesday for anz153.
Gale Warning from 7 PM this evening to 5 am EST Tuesday for