Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 02/13/21
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
643 PM EST Fri Feb 12 2021
Weak surface troughing evidenced by variable to nil surface flow
combined with diurnal heating has led to an area of light to
occasionally moderate snow from roughly Tuscola county straight
southward into Metro Detroit throughout much of the late afternoon.
There is low confidence in this activity as modeled ascent and UVVS
are generally non-existent. Carried a prevailing or TEMPO MVFR snow
group as activity continues to hold on. Potential exists for some
brief periods of 1SM snow. Frontal wave and frontogenesis brought on
by increased ageostrophic circulation from entrance region to jet
streak will lead to a period of system relative isentropic ascent on
304-309 Ke surfaces after 10Z Saturday morning. Potential exists for
light snow development at all tafs Saturday morning. Conflicting
signals on event with very good depth to supersaturation wrt to ice
and frontal inversion at 12.0 kft agl, however, modeled ascent/UVVS
remain very weak Saturday. Calling for IFR conditions in vsby
reductions and cigs.
For DTW...Potential exists for 3SM light snow activity at DTW,
however, confidence is low on likelihood and duration. High for cigs
less than 5000 ft agl.
.DTW THRESHOLD PROBABILITIES...
* High for ceilings at or below 5000 feet tonight and Saturday.
* High for ptype of snow Saturday.
Issued at 403 PM EST Fri Feb 12 2021
The inverted surface pressure trough lingering over Lower MI helped
light snow linger into the afternoon and continues supporting flare-
ups of snow showers late in the day. A diminishing trend to flurries
is expected before ending due to relaxation in the trough as the
surface pressure field becomes more influenced by the upstream wave.
Some lengthy breaks of sun have also occurred surrounding the
patches of snow lending some confidence for dry weather setting in
for most of tonight. The return and thickening of clouds provides a
floor to low temperatures generally in the 10 to 15 degree range by
Snow begins near the Ohio border around 5 AM as low level dry air
puts up the usual resistance to onset. However intensity ramps up
quickly as larger scale forcing sharpens vertical motion within the
mid levels of the frontal zone. Regional model solutions match up
well with satellite and radar across the Plains this afternoon
depicting primarily upper jet driven cloud enhancement associated
with bands of snow with several sub 1 mile visibility observations
in Nebraska and Iowa. The most intense snow is tied to a
strengthening of the mid level front that also matches up nicely
with a tightening of theta-e at 700 mb. This configuration of larger
scale forcing then slides quickly into the Great Lakes late tonight
and becomes enhanced by a lobe of short wave height falls trailing
the right entrance region of the upper jet which combine for the
quick increase of intensity during the 5 to 8 AM time period.
Examination of a 15Z RAP cross section does show some concern due to
a stable stratification of theta-e with height and a muted vertical
motion response leading to a shallower ageostrophic circulation.
This likely translates into cycles of intensity and shifting
position of bands during the event. Moisture remains adequate in the
1-2 g/kg range of specific humidity centered around 700 mb but with
only about 30 knots of flow. While a few hot-spots remain possible,
these characteristics ease concern for higher end snow accumulation.
The pattern otherwise passes the eye test for overachievement
especially given model soundings show a DGZ spanning the profile
from about 925 to 500 mb which is about as good as it gets for
dendrites and high snow ratio. Bottom line calls for a general 1 to
3 inch snowfall across SE MI likely with the first inch or two
falling from 6 to 10 AM and then grinding out another inch during
the remainder of the day.
System snow diminishes and ends Saturday evening as the wave of low
pressure sweeps into Canada. Its exit initiates some progression of
of the larger scale polar low aloft which allows a resurgence of
arctic high pressure into the Great Lakes Sunday and Sunday night.
The result is dry weather but a step down in temperatures through
Monday morning when readings end up in the single digits to near
zero in the typically colder locations. Wind chill trends continue
to be monitored as potential for -10 to -15 is in play Monday
The polar low aloft drifts eastward in the northern stream but not
far enough to deflect concern for an increasingly active southern
stream storm track that sets up Sunday into early next week. Some
phasing occurs which allows southern stream moisture to filter in
through the mid level frontal zone lingering over the Great Lakes.
This supports maintenance of chance POPs for light snow later Sunday
night and Monday which could result in light accumulation. The
primary southern stream trough/short wave then moves into the TN/OH
valleys with potential for a more substantial period of snow Monday
night into Tuesday. Southern Lower MI is more on the north flank of
this system which suggests holding POPs in the chance category while
the storm track is refined in upcoming forecasts.
Light to moderate northeast flow will continue into Saturday as a
weak low pressure system approaches from the upper Midwest and
traverses east across the central Great Lakes. The low pressure
system will bring a round of light snowfall to the local waters
while enhancing gusts to an extent over the north-central Lake Huron
basin up to 25 knots at times. Winds will trend variable in
direction briefly Saturday night in the wake of the low pressure
system and reorient northwesterly Sunday as high pressure becomes
reestablished over the Great Lakes. The flow veers to more of a
northeasterly direction for early next week and remains at moderate
strength as another low pressure system skirts by just off to the
southeast of the northern Ohio Valley.
Through well into next week, temperatures well below freezing will
continue to promote ice growth across the lakes. The ice growth have
already led to near or complete ice coverage of all local nearshore
waters of Lake Huron, Lake St. Clair, and western Lake Erie. This
trend is expected to continue through late February.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
You can obtain your latest National Weather Service forecasts online
Area Forecast Discussion
National Weather Service Hanford CA
340 PM PST Fri Feb 12 2021
.UPDATE...Updated air quality issues section.
Breezy conditions across Kern County today after a storm system
and cold front passed through. Another system will pass through
the district tomorrow, bringing even stronger winds to the wind
prone areas of Kern County, with light precipitation expected for
the Sierra Nevada. By early next week, a system may bring light
mountain snow before ridging settles in late next week.
Gusty wind conditions occurred this morning in the windy areas of
Kern County after the passage of a cold front, with a few areas
gusting over 50 mph. Breezy conditions are still occuring just
below the mountain passes of Kern County, and are expected to do
so according to HRRR guidance. A Wind Advisory will remain in
effect through this evening for the aforementioned areas. Another
system is right on the heels of today`s system and will begin to
impact the district late tonight. This system will be of the
inside slider variety, so precip amounts will be limited. That
said, a few inches of snow in the Sierra Nevada through late
The main impact with this system will be the wind. Model guidance
has really honed in on the trajectory of the jet aloft being
favorable for strong winds through the Tehachapi mountains and
passes, as well as the Mojave Desert. The NAM is showing a 100 +
knot 500 mb jet flowing orthogonal to the Tehachapi Mountains,
which could translate to wind gusts as high as 70 mph through and
below Kern County mountain passes by late morning or early
tomorrow afternoon lasting throughout the day. A High Wind Watch
remains in effect, and will likely be upgraded to a High Wind
Warning once today`s Wind Advisory expires.
Dry and cooler conditions are to be expected on Sunday under cool,
northwesterly flow aloft. An impulse like system comes through
the northwesterly flow on Monday, which may trigger another round
of wind through Kern County, with only scant precip amounts for
the Sierra Nevada forecast. The rest of next week looks to calm
down weather-wise as high pressure in the Eastern Pacific builds.
Blended model guidance indicates a warming trend is possible next
week to accompany the high pressure pattern.
Across the Tehachapi Mountains, mountain obscuration/IFR
conditions will continue for the next 24 hours. Wind gusts to 35
knots over the Tehachapi mountains for the next 18-20 hours,
increasing to over 50 knots by 20z Saturday. Otherwise, VFR
conditions will prevail across the San Joaquin Valley.
.AIR QUALITY ISSUES...
The level of certainty for days 1 and 2 is high.
The level of certainty for days 3 through 7 is medium.
Certainty levels include low...medium...and high. Please visit
www.weather.gov/hnx/certainty.html for additional information
an/or to provide feedback.
Wind Advisory until 9 PM PST this evening for CAZ195>199.
High Wind Watch from Saturday afternoon through late Saturday
night for CAZ195>199.
On Friday February 12 2021... Fireplace/Wood Stove Burning Status
is: No Burning Unless Registered in Fresno... Kern and Madera
Further information is available at Valleyair.org
Area Forecast Discussion
National Weather Service Key West FL
1043 PM EST Fri Feb 12 2021
The 00z KEY sounding showed a deep layer of southerly flow aloft,
from 1,500 feet up to about 15,000 feet. The mean 1000-700 mb
layer flow was southerly at 15 knots. This direction and strength
is typically favorable for bringing the leftover environment from
afternoon Cuban convection northward across the Straits during
the night. Indeed, convective cells have become persistent over
the distant Straits north of Varadero, Cuba. CAMS guidance shows
good consensus in further blossoming convective coverage overnight
while moving cells north toward the Keys. The HRRR forecast
lightning parameter shows a developing chance of thunder over the
eastern Straits in the next few hours, possibly approaching the
Middle and Upper Keys during the near-sunrise hours. Have
therefore increased PoPs through Saturday morning over the
Middle/Upper Keys and the adjacent oceanside waters.
During the day on Saturday, the nocturnal area of convection will
exit northeast of the Upper Keys by mid-day, leaving a dry
afternoon for the Keys.
Have split off the eastern Straits to add cautionary headlines
there, due to guidance indicating 15-20 knot winds there through
Saturday morning. This split also captures the higher rain chances
through Saturday morning over the eastern Straits, along with the
potential for isolated thunder.
Otherwise, little change in the overall pressure pattern is
forecast for the Keys thorugh Monday night.
From synopsis...Gentle to moderate southeast breezes will prevail
across the Keys coastal waters through Tuesday, as the waters
remain between Atlantic high pressure south of Bermuda and lower
pressure over the central Gulf of Mexico. A temporary easing is
possible on Tuesday as a cold front washes out and dissipates over
the eastern Gulf. Southeast breezes will freshen on Wednesday in
advance of a deepening surface low in the western Gulf of Mexico.
Cloud cover is in the process of increasing as the same
environment that supported afternoon convection over the Keys
moves north across the Straits during the nighttime hours. Radar
already shows heavy showers 60-80 miles south of MTH, and these
showers will move north and expand in coverage overnight. Showers
will affect the vicinity of the terminals during the near-sunrise
hours, with sufficiently high confidence at MTH to include a TEMPO
for heavy showers.
During southerly flow aloft, afternoons become reliably rain-free
over the Keys as convection instead focuses to our south over
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Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
926 PM CST Fri Feb 12 2021
924 PM CST
The main updates this evening were to attempt and refine PoP
trends and overall QPF/snow amounts tonight and into Saturday
00z RAOBs out of Davenport and Lincoln sampled quite a bit of
lingering dry air, with the near-surface dry layer initially noted
on the DVN sounding likely having saturated quite a bit since
then. Mid-level dry air was much more prevalent on the ILX
sounding, and with cloud bases still at or above 10,000 ft into
north-central Illinois, this is likely still the case. Given the
overall modest nature of ascent overspreading the area, have
trimmed PoPs quite a bit through tonight for areas south of a
roughly Pontiac to DeMotte line with it unlikely that light snow
will be able to chew through this dry layer. Have done the same
across far northern Illinois and to the state line for similar
reasons. In between these two areas, broad/disorganized f-gen will
continue to develop overnight as the exit region of an
anticyclonically-arcing jet streak pushes in overhead, and this
appears to be where the main focus will be for steady (mainly
light) snow overnight and into Saturday morning. Given the rather
lazy nature to the ascent, thinking that snow-to-liquid ratios
may end up a smidge lower than you`d otherwise expect given the
cold thermal profiles and deep dendritic growth zone...perhaps in
the 15-20:1 range.
The main axis of snowfall amounts in the 2-4" range has shifted a
bit north, focused more around the I-80/88 corridors with this
update, with amounts lowered to for our southernmost counties to
an inch or less. PoP trends for Saturday morning are a bit
unclear, as we`ll begin losing the better jet support, resulting
in an overall weakening to the already modest mid-level f-gen.
This may allow lingering light snow near/south of I-80 to
dwindle, before additional ascent from an incoming shortwave
pushes across our northwestern locales. Have trimmed PoPs south
and east of I-57 as a result, and left the categorical/definites
Lake effect is the next challenge. Near-surface convergence is
tightening up near the lake, with winds turning northwesterly at
the Crib, and veering more easterly across southwestern Michigan.
A single lake band is evident along the western Michigan
shoreline, and is starting to push west. Guidance like the 3-km
NAM and RGEM, which seemingly handled yesterday`s LES event fairly
well, continue to crank up quite a bit of QPF across far northern
Porter county, and have been joined by the Hires WRFs. Have
nudged snow amounts up in this localized area with some amounts
approaching 5-6" near the lake in the latest forecast. Not
entirely convinced of the double-digit output from some of the
guidance given the altogether marginal LES parameters, but we`ll
keep an eye on things. Either way, we`ll be continuing the Winter
Weather Advisory for Porter County, and could need to sneak it
into Lake County as trends become better established.
234 PM CST
Through Saturday night...
Currently, an elongated 130-150 kt jet streak extends from eastern
Minnesota to southern Ontario. In the right entrance region of the
jet streak is a similarly elongated zone of radar echoes stretching
from central Nebraska though northern Illinois. Recent AMDAR
soundings off aircraft departing Chicago terminals depict a short
but very dry layer centered at about 5000 feet, which is eating any
and all snowflakes on their trip to the ground. (So, most of those
aforementioned radar echoes are actually virga). The dry layer is
expected to remain in place through the remainder of the afternoon
and evening hours, acting to delay the onset of snow compared to
Using the RAP 850 mb relative humidity as a proxy to track the
elimination of the dry layer, it appears light snow should start
trickling down to the ground after dark and in the 9-11 pm window.
Thereafter, steady, light snow will continue through Saturday
morning. A deep snowflake growth layer of nearly 15,000 feet will
afford fluffy snow ratios of 20:1 to 25:1, with gradual lift
supporting a snow rate of 0.1 to 0.3"/hr. Upper-level lift will then
wane after daybreak Saturday as the jet streak scoots eastward,
though light snow showers will likely persist through much of
afternoon. Generally 1 to 3" of fluffy snow looks on track with the
highest amounts in the vicinity of I-80. Locally higher totals are
possible along the Lake Michigan shore of Porter county, Indiana
where an overnight land-breeze-forced lake effect snow band should
boost snow rates after midnight. The inherited Winter Weather
Advisory for Porter County thus remains appropriate. Regardless of
how much snow falls across the area tonight/Saturday, untreated
roadways may become slippery.
Snow will end quickly Saturday evening and clouds will attempt to
clear from west to east. With a fresh snowpack in place,
temperatures overnight into Sunday should drop below zero areawide.
Winds of 10 to 15 mph will make it feel more like 20 to 25 below
zero, especially west of I-39. Not much reprieve is expected Sunday
with highs in the single digits and wind chills of 5 to 15 below
zero. Such cold temperatures can lead to frostbite in as few as 20
minutes, so bundling up from head to toe in multiple layers is a
must if planning to head outdoors.
222 PM CST
Sunday through Friday...
Lake effect snow will likely continue Saturday night into early
Sunday across Porter and possibly Lake counties of IL as 925 mb flow
remains NE for a time on Sunday.
Sunday into early Tuesday... The anomalously lower heights in the
mid-levels with the aforementioned lobe of polar air, with 500 mb
values of 505 dm, will shift east/southeastward during Sunday into
Monday. This influence is more so Monday afternoon into Monday
night, with a more stout disturbance progged to eject northeastward
from Texas/southern Plains toward the Ohio Valley. But there are
still signs even late Sunday through early Monday the baroclinic
zone around 700 mb tightens over the southern CWA with some
saturation and possible light snow. There is some variance in
guidance on specifics as far as how far north the light snow
footprint may extend, still with our I-55 and southeastward corridor
While forecast adjustments from the blend were to still boost
PoPs along and south of I-80 for mainly Monday afternoon and
night, the signal for how widespread the influence of even this
stronger second system is not quite as robust. Cyclogenesis is
forecast over the southeastern U.S. tracking into the Appalachians
region and supports the best chances for some snow to inch
northward over our area. Again our highest chances remain along
and southeast of I-55, but even light snow will be fluffy and
could create impacts across the entire area. The potential is
there for a wind chill headline going through Sunday and into
Monday morning, but this may be confined to areas across north
central IL and west of Chicago. Still will be solidly below zero
and in some places well below to start the week on Monday morning.
Lake effect snow Monday into Tuesday...Lake effect snow during this
time may be ongoing through these several events, and there are
hints at the development of decent convergence zone/associated
surface low, in northwest Indiana Monday, and then shifting into
northeast IL later Monday and Monday night. There could be some
lingering snow/flurries Tuesday morning, but this period is of lower
confidence than Monday night.
Tuesday and beyond....
The strong arctic high will ever so slowly, but optimistically (for
those of you just looking for something to hang your hat on) lose
its grip on the region, and we will inch our high temperatures back
into the 20s mid week. Before we finally say good riddance to the
core of the arctic air, another system is currently advertised to
follow a similar Texas to Tennessee valley to the Appalachians
track late in the week. This could still graze our area with some
light snow before the weakening ridge axis finally makes its way
overhead toward week`s end and we shift the low level low back
around to the southwest for the first time in quite a while.
For the 00Z TAFs...
539 PM...Primary forecast concern is light snow developing this
evening and continuing through Saturday afternoon, along with
Light snow is expected to develop across the area by mid/late
this evening and continue through at least late Saturday morning
and possibly through mid afternoon. A narrow band of snow is
expected to develop and then expand in coverage and there remains
uncertainty regarding exactly where and when this will occur but
much of the latest guidance shows light snow developing along a
vys/lot/gyy line by mid evening and then expanding north through
early Saturday morning. Current forecast has this trend covered
well and made no significant changes. While prevailing 1-2sm vis
looks on track, through midday Saturday, there may be a corridor
of moderate snow that may reduce vis under 1sm but confidence for
location remains low and no mention of lower vis with this
forecast. Cigs are expected to lower into mvfr soon after snow
begins with several hours of low mvfr expected through early
Saturday afternoon. Ifr cigs will be possible if and where any
moderate snow develops. As the snow tapers off Saturday afternoon,
cigs will lift through mvfr with scattering/clearing expected
North/northwest winds around 10kt will continue through the period
with speeds increasing to 12-15kts Saturday afternoon along with
some gusts into the 15-20kt range possible Saturday evening. cms
IN...Winter Weather Advisory...INZ002 until 6 PM Saturday.
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