Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
740 am EDT Monday Mar 30 2015
a cold front moving east from the western Great Lakes will bring
a chance of rain and snow showers to the north country on Monday.
After a brief cool down Tuesday and Wednesday...a warming trend
will lead to temperatures in the middle 40s to low 50s by the end of
the week. Late in the week a low pressure system will bring a
chance for mostly rain showers.
Near term /until 7 PM this evening/...
as of 713 am EDT Monday... the atmosphere over northern New York
has finally become saturated enough in the low levels to allow the
snow to reach the ground. Mss/slk/ogs have all reported snow as
in the last hour. So the increase in probability of precipitation over from 3 hours ago
looks pretty good and no new changes were done. Over Vermont its
still going to take a bit more time to saturate the low levels so
the radar echos right now are most likely still virga. Temperatures were
running a bit low so I brought temperatures and dew points more in line
with current observation.
Previous discussion as of 356 am...comparing current radar and
WV/infrared analysis and the ground truth provide two apparent different
solutions to what is currently going on. At the surface, the low
level moisture is still quite dry from the high pressure pressure
and weak upper level ridge over the weekend. So the relatively
high radar echoes and colder brightness temperatures from the WV
satellite would generally mean the snow should be reaching the
ground but it isn't at the moment. That said from current surface
observation across northern New York the precipitation that is falling is
moistening up and priming the low levels of the atmosphere such
that later today when the rest of the surface front gets here
there should be ample moisture in the low levels to generate the
rain and snow showers we've been expecting. Based on the dynamic
forcing of the upper level low not being too strong expect the
most quantitative precipitation forecast and snowfall totals to be in the higher terrain
benefiting from terrain based ascent. Quantitative precipitation forecast numbers over the region
will generally range from 0.1" to 0.3" in the higher terrain above
1500 feet with only really a trace to 0.1" expected in the
Champlain and Connecticut River valleys. That corresponds to
snowfall totals ranging from a dusting in the Champlain Valley to
2- 4 inches at the summits.
Based on local Froude number calculations through around 15z
today winds will be slow moving to nearly blocked which supports
the idea that the early snow fall will most likely fall over the
western slopes before Froude numbers rise above 1 and end up with
favourable snow regions becoming The Summit and the Lee side of
the mountains. Currently the guidance has been running 2-4 degrees
too cold which has cause most of the model soundings to be
forecasting mostly snow with this event however being that its
most likely a boundary layer temperatures ptype event I expect
that there will be enough warming in the boundary layer to melt
all of the snow and be mostly rain by 18z. That however as the
front moves closer and we get more west to northwest upslope flow
we will see cooler temperatures move back into the boundary layer
and transition back to all snow by 21-00z this evening. Some of
the guidance is forecasting a weak dry slot to move into the
Champlain Valley so its quite possible that in the valley we wont
see much of any rain or snow accumulation if any. With the
southerly flow most of the morning and early afternoon expect maximum
temperatures to warm into the middle to low 40s across the forecast region
Additionally... forecast soundings show that as the front moves
through, lapse rates will quickly steeping leading to gust front
mixing to the surface and producing a quick burst of wind ranging
from 30-40 kts. Those steeper lapse rates will cause gusty winds
to be present during any of the showers be they rain or snow so
some caution should be taken although nothing is expected to be
too significant in terms of gusts.
Short term /7 PM this evening through Wednesday/...
as of 356 am EDT Monday...Tuesday behind the surface front an
upper level trough will pass through the region with north
westerly flow. Any leftover orographic snow showers or flurries
over the Adirondacks and The Greens will be ending. A weak short
wave will develop and run quickly across New York and pennsylvaniawhich
will further the north northwest flow bringing more cold air
advection into the region. However the precipitation should remain well
to the south of the forecast region. 850 temperatures will be running
about -9c on on Tuesday which will will stay on the cooler side
running about 5 degrees colder than today.
Tuesday night light northwest flow continues low to middle level
cold air advection as a very broad dome of high pressure moves
over the Great Lakes. We wont be quite as dry as yesterday and
there might be enough wind in the boundary to offset the otherwise
decent radiational cooling conditions. Expect the normal cold
prone areas to drop into the low single digits with the Champlain
and Saint Lawrence valleys both holding in the in the teens to
Wednesday the upper level trough will start to slide out of the
region leaving behind dry air under northerly low level flow
shifting to more westerly by middle afternoon. The weather will be
quiet and dry with a chance to see some sunshine and temperatures running
on the cooler side in the middle 30s.
Long term /Wednesday night through Sunday/...
as of 352 am EDT Monday...a fast/progressive middle to upper level flow
regime is expected to prevail across the northern Continental U.S....bringing
changable conditions to the north country during the extended forecast
A surface ridge and 700mb shortwave ridge will be in place to start
the period Wednesday night. Generally looking for tranquil
conditions...though middle-upper clouds will likely be on the increase
ahead of a warm front advancing northeastward across portions of western PA/western
New York and southeastern Ontario. While precipitation is not expected...clouds
will likely mitigate radiational cooling...and temperatures should
become quasi-steady after midnight. Still expecting temperatures below
seasonal levels for the start of April...with lows mainly in the
upper teens to lower 20s. May see a few readings 10-15f across the
Northeast Kingdom of Vermont...with anticipated later arrival of
Warm frontal passage is anticipated Thursday from SW-NE with mostly
cloudy conditions. May see an early rain/snow mix...but generally
looking at light/scattered rain showers with 850mb temperatures warming to
+3 to +5c by late Thursday afternoon. Surface temperatures will likewise be
trending warmer...with afternoon highs in the middle 40s to lower 50s.
Some timing differences between the 00z European model (ecmwf)/00z GFS with
developing fast SW flow developing late Thursday and Thursday night.
Embedded shortwave in fast SW flow likely brings another period of
rainfall either Thursday afternoon or Thursday night. Carried chance probability of precipitation
for now with timing uncertainties.
Shallow cold front shifts slowly southeastward Friday...but with 850-500mb
flow remaining southwesterly...Don/T foresee any significant cooldown during
the daylight hours Friday...despite west-northwest surface wind shift. Likely still
seeing highs in the middle 40s to upper 40s. Extended period of above
freezing temperatures should result in further river ice breakup...but
limited quantitative precipitation forecast (generally less than 0.50") will keep river flows
relatively low and should preclude any flood threat.
Both 00z GFS/European model (ecmwf) suggest frontal zone stalling near or just
southeast of the area Friday night into Saturday...with potential
development of a wave low along the boundary. This next system could
bring potential rain/snow mix...changing to all snow as precipitation
potentially intensifies later Friday night into Saturday morning.
May see some travel impacts due to several inches of wet snow
accumulation...and will need to monitor Friday night/Saturday period
for this potential. Clearing and seasonably Cold Saturday night into
Sunday. Lows Saturday night upper teens to lower 20s...followed by
highs in the 30s for Sunday with dry conditions expected.
Aviation /12z Monday through Friday/...
through 12z Tuesday...a trough of low pressure approaching from
the Great Lakes will bring MVFR/VFR ceilings and intermittent
periods of light rain and snow showers to the taf sites through
the period. Moderately strong pressure gradient will continnue to
result in moderately strong S-SW winds this morning...with
sustained winds generally 10-15 kts and gusts 25-28 kts. Can/T
completely rule out low level wind shear...but low-level mixing will generally be
sufficient to reduce magnitude of vertical shear and get
moderate/gusty winds to the surface...as indicated in tafs.
Winds will gradually shift into the SW and west with surface
trough passage this afternoon into this evening. Associated
showery precipitation is generally not expected to be
significant. Best chance for snow shower activity will be at
slk/mpv this morning...and again after 22z this afternoon as
ororgaphic ascent increases with northwest winds. Should see periods of
MVFR conditions at slk/mpv...while ceilings and visibility generally
remain VFR at the other taf locations. Generally sprinkles and
rain shower activity at the other taf sites with warmer low-level
conditions. Will see a lowering of ceilings from near 6-7 kft
during the pre- dawn hours...to 4-5 kft during the daylight hours
and 2-4 kft tonight. Hir terrain will become obscured at times during
the daylight hours Monday...and through Monday night.
Outlook 12z Tuesday through Friday...improving to VFR Tuesday and
then fair/VFR with light winds Tuesday night through Wednesday
night with surface high pressure in control. Next frontal system
brings potl S-SW wind gusts 25-30 kts during Thursday
night...followed by MVFR/intervals of IFR in periods of rain late
Thursday night through Friday.