Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/22/16
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
547 PM EST Wed Dec 21 2016
No aviation issues through the first half of the night, simply a
blanket of mid cloud emanating from low pressure tracking through
northern Michigan. Low cloud development will require the arrival
of a trailing cold frontal passage and subsequent low level
saturation expected to commence through the early-mid morning
period. Upstream observations across WI/MN suggest a slower arrival
time, greater potential for conditions to slip into MVFR now
centered 09z-12z. MVFR stratus tied to low level thermal troughing
likely persists well into the day Thursday. Modest southerly wind
shifts to westerly with the frontal passage.
For DTW...entry of a lower stratus now centered 09z-12z. MVFR
conditions expected thereafter into Thursday afternoon.
//DTW Threshold Probabilities...
* Moderate for ceiling falling below 5000 ft Thursday morning.
Issued at 323 PM EST Wed Dec 21 2016
Vertically stacked upper wave and surface low pressure over the
Upper Midwest still on track to cross Michigan tonight. Weak
isentropic ascent so far today has yielded widespread returns on
radar but little reported snowfall yet as ascent is still working to
overcome dry air observed on 12Z DTX Raob. Isentropic ascent will
ramp up very late this afternoon and this evening as the low draws
closer and southerly flow increases. This will provide a better
chance for light snow to reach the ground, mainly over the Saginaw
Valley and Northern Thumb where better forcing and more low-level
moisture will reside. Accumulations, if any, will be very light due
to fast-moving nature of the system and the dry air preceding it.
Would expect to only see a few tenths of an inch of snow at best by
midnight. QPF amounts are supported by upstream observations in
Wisconsin picking up between a trace and a few hundreths of precip.
Locations south of I-69 stand to see only a small chance for a
light snow shower or some flurries.
Moisture in the dendritic growth zone strips away after midnight
behind an occluded frontal boundary. Forecast concern then turns to
moisture depth under an inversion, and whether or not we can
generate any freezing drizzle. NAM is the most aggressive with low-
level moisture, as typical, and will favor the drier RAP and HRRR
and leave mention of drizzle out of forecast late tonight through
Thursday morning. Clouds and around 10 knots of wind persisting
overnight combined with only weak cold air advection should hold
temperatures in the mid to upper 20s.
Upper ridge centered over the Central CONUS will slide closer to the
Great Lakes tomorrow, allowing surface high pressure to begin
expanding into the area. This should provide dry weather, although
clouds are expected to remain locked under an inversion into the
afternoon until some drier air is entrained into the boundary layer.
Deeper mixed layer tomorrow will provide slightly warmer
temperatures despite some cooling low in the column. Will favor max
temps in the mid 30s to near 40.
Next potentially impactful item to watch will be a good chance for
quick but light snow accumulation Friday night. Energy from the
upper low west of Baja California will eject northeast and track
across Michigan late Friday into Friday night. Latest model runs are
consistent with the system pulling some of the low-level moisture
now streaming across Baja into Michigan Friday night as the surface
low tracks across Northern Michigan. Track looks far enough north
for a rain/snow mix to be possible over the far southeast corner of
Michigan as warmer air is drawn into the Southern Great Lakes. Will
mention a rain/snow mix south of I-96 for now, with the caveat that
some fine-tuning of precip type will be needed as the system draws
closer. Precip will occur ahead of a cold front, and event should be
a fairly quick one, lasting 6-8 hour at most in any location. Cross-
sections from NAM/GFS show a layer from the surface up through 800-
700mb of 3-4 g/kg of specific humidity, even in the coldest air
(MBS). These models also suggest a lower snow-ratio event that only
generates about an inch of snowfall at best (where precip remains
all snow). Given moisture origin and content, cannot discount higher
QPF amounts on the Euro/GEM up to 0.25 inches. Snow amounts could
need an upward adjustment if higher QPF starts to look more likely,
or if thermal profiles trend cooler.
Ridging building over the Great Lakes region on Saturday in response
to amplifying trough over the Western US will yield milder, drier
conditions. Strong low pressure then develops ahead of the trough
over the Central Plains on Sunday before moving northeastward over
Minnesota Sunday night. While Sunday will start with seasonable
temperatures in the 30s, warm air advection will overspread Michigan
Sunday night into Monday. As a result, rain/snow mix moving into
Michigan on Sunday night will transition to rain until the frontal
passage on Monday. As the low moves away from the Great Lakes on
Tuesday, high pressure builds in bringing drier but colder
conditions with highs returning to the low 30s.
A weakening low pressure system will track east across Lake Superior
tonight, pulling an occluded front through the eastern Great Lakes
tonight after midnight. This system will produce some light snow
showers in addition to briefly enhanced winds. Looking at the
current wind field associated with the low and taking into account
the weakening trend, it looks like the models are handling the winds
over the lakes pretty well. Wind gusts should peak around 20-25
knots right behind the front with cooler air advecting in upon
northwesterly flow on Thursday bringing another wind maximum. This
system will lift northeast as strong high pressure system slides
across our south toward the mid Atlantic states by Friday. The next
system will lift through the western Great Lakes on Friday night and
Saturday putting our area in the tighter gradient in between the two
systems. Increasing stability should keep the potential wind gusts
tempered below 30 knots.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
You can obtain your latest National Weather Service forecasts online
Area Forecast Discussion...UPDATED
National Weather Service Gray ME
1054 PM EST Wed Dec 21 2016
An Alberta Clipper will swing through the region on Thursday
bringing light snow accumulations throughout the region. Behind
this system...high pressure will bring dry and seasonable
conditions to end the week. Another fast moving weather system
will bring a potential round of snow or mixed precipitation on
Saturday with seasonably mild temperatures and dry weather
expected for Christmas and into Monday of next week.
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Update...Again no major changes to the current forecast. Have
tweaked temps up some in the near term...as high clouds invading
from the W are limiting cooling at the moment. Low temps should
eventually come as a result of evaporational cooling as precip
tries to break out near sunrise. Taking a peek at upstream
obs...snowfall seems on pace with the latest HRRR forecasts. These
get snow into Wrn zones by 10 or 11z. This matches likely PoP in
our forecast well...so I see no need to make any major
adjustments there. Snowfall amounts are also in line with the
higher end of modeled QPF. I could see a slight increase in QPF
across the forecast area to around 0.15 to 0.20 inches. Overall
though this is not a significant increase in snowfall amounts
forecast for the area.
There are some mixed signals from mesoscale model guidance this
afternoon. The highest resolution sources favor the largest
accumulations across central NH and the White Mtns. The lower
resolution versions tend to place that snowfall across Srn NH.
There is good agreement of QPF however...with around a quarter
inch looking like the maximum output for this fast moving wave.
That leaves the forecast in good shape with 1 to 2 inches and an
isolated higher total in the mtns.
Previous discussion...High Impact Weather Potential: Light snow
develops towards daybreak.
Current Pattern: Early afternoon water vapor imagery shows broad
troughing over the eastern half of NOAM....with one very moisture
starved shortwave over New England as of this writing...with a more
impressive...yet still moisture-starved shortwave upstream moving
into the Great Lakes region. Between these features is a region of
shortwave ridging...with an attended high pressure ridge axis at the
surface. This feature will control our weather through much of the
overnight before the flow backs ahead of the upstream shortwave and
top-down saturation commences...with some snow likely to develop
over western locations by the very end of the near term forecast
Through this evening: Initial shortwave is now passing east of our
longitude...and had been responsible for a build up of clouds
upstream of the mountains of New Hampshire and western Maine...with
a few snow showers...but little else. I don`t expect much in the
way of changes through this evening...with clear skies south and
east of the mountains allowing temperatures to start dropping
rapidly from their mild afternoon highs in the 30s to around 40 back
into the 20s to around 30 by 7pm. Afternoon westerly winds will
diminish markedly by sunset.
Tonight: Height rise center associated with shortwave ridge axis
moves though the region for the first half of the night before warm
advection and associated top-down saturation develops.
Thus...expect temperatures to drop quickly in the evening /with some
bust potential on just how low they go...but some teens possible/
before leveling off and likely rising towards daybreak. Antecedent
airmass is dry...but not terribly so...so have no reason to stray
much from model consensus arrival time of precipitation of 4-6am over
NH...moving after the daybreak for all but extreme western Maine.
Temps certainly cold enough for snow all locations through
daybreak. Less than one inch of snow by daybreak over NH is
.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
High Impact Weather Potential: Period of light snow Thursday.
Pattern: Great lakes shortwave sharpens somewhat as it moves
overhead during the day Thursday...with a very weak surface
reflection deepening once QG forcing for ascent reaches the waters
of the Gulf of Maine. While this coastal low will strengthen
rapidly...it will be too little too late for much of a significant
impact over our forecast area. However...a period of warm advection
Thursday morning in the presence of -DIVQ will yield a period of
light precipitation for much of the day. Shortwave ridging and
associated surface high builds in Thursday night...with quiet
weather to end the short term forecast period.
Thursday: Upstream clipper will be located near Georgian bay
Thursday morning before moving into northern New England by
afternoon with a new low getting it/s act together towards evening
in the Gulf of Maine. Upper forcing for ascent is modest...with
-DPVA/height falls associated with positively tilted
shortwave...but poorly placed jet and limited moisture paints a
picture of generally light precipitation due primarily to
QPF/Snow: Given very meager PWATs to work with /generally less than
one third of an inch/...and fast moving nature of clipper...this will
not be a prolific precipitation producer. Model consensus favors
about 0.10-0.15" liquid...which seems very reasonable...with about
6 hours of precipitation expected for any location.
Vertical profiles do suggest decent snow:liquid ratio potential...
with vertical motion centered within and just above the dendritic
growth zone. Cobb algorithm suggests ratios near 14-15:1 away from
Ptype: As indicated by previous shift...likely that nearly all
precipitation that falls will be snow. Wet bulbs near 34-35F along
the coast by late morning suggest that some rain may mix in...as
precipitation comes to an end...limiting coastal accums a tad.
A look at SREF and NCAR ensemble plumes yields a few members up to
3" over inland areas and see this as the upper limit for snow.
Timing will be problematic for the commute...with all of NH and some
of western Maine likely seeing snow begin by 7am. Will continue
mention in the Hazardous Weather Outlook and could foresee a SPS down
the road...but no headlines warranted given these amounts.
Thursday Night: High pressure builds into the region with building
heights aloft arriving after midnight. Cold advection behind
Thursday/s low will be rather meager...with a seasonably mild
overnight expected. Good gradient through much if not all of the
night with 1000 mb geostrophic winds remaining 15-25kts...which
should keep all but the more sheltered valleys coupled.
Thus...expect a few spots to drop into the upper teens...with 20s
for most locations. If the wind dies off a bit sooner than
expected...more widespread teens are likely.
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Models are in decent agreement on the longwave pattern through
the end of the calendar year. We begin the period with a fast low
amplitude flow. However...digging shortwave energy entering the
pacific northwest will quickly carve out a deep trough over the
western third of the CONUS for the start of the upcoming weekend.
Downstream we`ll initially see rising heights and a warming trend
and rather quiet weather for the holiday weekend. By early next
week...we`ve transitioned to a high amplitude progressive pattern
that portends a stormy end to 2016. In the dailies...high pressure
builds by to our south Friday and Friday night with quiet weather
and seasonable temperatures. On Saturday...a passing shortwave
will drive an associated cold front across the area with a few
snow and rain showers with p-type driven by boundary layer
temperatures. Rising heights take over with a ridge of high
pressure traversing the area for Sunday into early Monday. A
potent shortwave and associated surface low will track from the
northern plains and western Great Lakes on Monday into eastern
Canada Tuesday. The warm front from this system will bring some
light overrunning precipitation to the area late Monday and Monday
night followed by a cold front early Tuesday. In the wake of this
system...gusty winds along with some upslope snow showers across
the higher terrain for Tuesday and Tuesday night. Rising heights
and a ridge of high pressure arrive by midweek. A deep trough and
associated storm system arrive for the tail end of the work week
with a mixed precipitation event for the region followed by
sharply colder temperatures and a few mountain snow showers to
ring in the new year.
.AVIATION /04Z THURSDAY THROUGH MONDAY/...
Summary: A fast moving area of low pressure will bring deteriorating
flying conditions for the day Thursday before high pressure moves
back into the region Thursday night.
Restrictions: VFR through the first half of tonight. Snow arrives
over NH terminals by daybreak Thursday with a roughly 6 hour period
of LIFR/IFR vsbys that moves east with time. VFR conditions return
Thursday afternoon/evening and continue through Thursday night.
Winds: Westerly winds around 10kts this afternoon will go
light/variable overnight and remain less than 10kts through the day
Thursday from the southwest. Winds shift northwesterly Thursday
LLWS: Not expected through Thursday Night.
Sat...Areas of MVFR in -shsn/-shra.
Mon PM - Tue AM...MVFR with areas of IFR -ra/-pl/-sn.
Winds/waves will continue to subside through this evening...then
all is quiet through Thursday as a clipper system moves over the
region. This system will spawn a stronger low over the Gulf of
Maine that will strengthen as it departs the region Thursday
night. This will bring renewed SCA conditions over the outer
waters due to northwesterly winds.
Fri...Small Craft conditions possible outside the bays.
Sat...Gale force winds are possible.
Sun...Small Craft conditions possible.
Mon PM - Tue...Gales force winds are possible.