Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 10/31/21

Area Forecast Discussion

National Weather Service Cheyenne WY

 652 PM MDT Sat Oct 30 2021

.SHORT TERM...(This evening through Sunday)
Issued at 247 PM MDT Sat Oct 30 2021

Lots of concerns with this forecast package.

Latest visible imagery was showing the MVFR ceilings continuing to
build south in response to the cold air advection settling over
the area. Meanwhile, the freezing drizzle/fog will continue to
develop overnight in response to low level warm air advection
and upslope flow. The low level warm advection will also set the
stage for potential instability overnight and Convective Symmetric
instability (CSI) release above 650mb. The HREF solutions are all
picking up narrow lines of E-W convection developing late tonight
across portions of the northern panhandle and Niobrara and
Converse county in Wyoming. Hard to say where these lines may
develop, but if they do develop they will be quite intense with
visibilities less than 1/2 mile at times. This snow in combination
with the fog and freezing drizzle caused us to lean towards a
Winter Weather Advisory from midnight through 00Z Monday.

The next concern is the dense fog potential near the I-80 summit
and eastern foothills mainly after midnight. This fog will most
likely a result of cold dense air and low level upslope flow. This
fog will most likely linger through the day tomorrow and even
spread further east towards Cheyenne and the southern panhandle.
As a result, we went ahead and issue a Freezing Fog Advisory for
the I-80 summit and foothills to get things started from 06z
tonight through 00Z Monday and let the evening shift expand if
necessary.

The next bigger concern is the shortwave moving into the region
Sunday into Sunday night. This feature will set the stage again
for some more potential instability and CSI banding. Only this
time these lines of convective snow bands may be a little further
south closer to the I-80 corridor. This period will certainly be
worth keeping an eye on and may produce some impressive snowfall
rates Sunday afternoon into Sunday evening. Stay Tuned.

.LONG TERM...(Sunday night through Saturday)
Issued at 247 PM MDT Sat Oct 30 2021

Unsettled pattern emerges through next week, with below normal
temperatures advertised. Long range model guidance has been
reasonable over the last few runs, with a deep upper level low
pushing eastward just north of the Great Lakes and an additional
upper level low coming onshore from the eastern seaboard. Between
these two systems, a semi-stationary upper level ridge sets up over
the Southwest, keeping the region under northwesterly flow and
dragging our temperatures to below normal. Several shortwaves are
expected to propagate within the flow through next week, with a deep
shortwave trough pushing eastward across the central Rockies Tuesday
into Wednesday. Decent moisture advection with this pattern, and a
bulk of the precipitable water values stretched across most of
Wyoming and into the Nebraska Panhandle with much drier air to the
north. Started trending temperatures downward on Tuesday with
forecast 700mb temperatures below 0C, indicating a likelihood for
snow west of I-25 and mostly rain to the east. Main concerns exist
across the domain that if the precipitation type starts out as snow,
can enough daytime heating occur to bring the temperatures up,
keeping precipitation type to all snow into the Panhandle. Overall,
confidence remains moderate with there being enough lift for
precipitation to occur, but lacks confidence in precipitation type.
Will need to evaluate later model runs to determine exact impacts.
As we transition into late-week, the upper level ridge is expected
to gain some momentum and push further eastward into the Central
Plains late Thursday night. While under the influence of this
pattern, precipitation chances begin to dwindle, outside of the
higher terrains. Most long range models have picked up on this
feature and adequately trended POPs downward, with little
modification needed through Friday. Dry conditions continue through
the weekend, with near zonal flow developing behind the upper level
ridge and a decent warmup expected, especially east of the
Wyoming/Nebraska border.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 648 PM MDT Sat Oct 30 2021

A difficult forecast ahead for tonight and into the day tomorrow as a cold
front treks south across the region. From now until roughly midnight,
conditions should be at their best and then quickly deteriorate after that.
For the evening/first part of the night, conditions should be VFR to MVFR.
Sites upstream from our CWA behind the cold front also are reporting a mix
of VFR/MVFR conditions. At the moment, all sites except KCYS are
VFR. If flight categories were to drop during this part of the
night, it would be due primarily to low ceilings. However, we
cannot rule out the possibility of some patchy fog during the
first part of the night since the lowest layers of the atmosphere
are pretty moist. If fog were to occur during this time, expect
flight categories to drop. For the second half of the night, after
midnight, conditions worsen. Model soundings show a fairly decent
saturated layer below 600MB and between -10C and 0C. This leads
to the potential for widespread freezing fog and drizzle. Included
BR in the TAFs to hint at dropping visibilities. Ceilings should
also lower with the drizzle/freezing drizzle which will be the
dominate factor in worsening flight categories. Models prog flight
categories to drop into IFR conditions, especially for the
Wyoming terminals tomorrow morning. Would not rule out LIFR
conditions, especially with the combination of fog/drizzle.
Because freezing fog/drizzle is a concern tonight into tomorrow,
icing of aircraft is definitely a threat since supercooled liquid
water will be present. There is still uncertainty with the
overnight forecast so will continue to amend TAFs as required.

&&

.FIRE WEATHER...
Issued at 247 PM MDT Sat Oct 30 2021

No critical fire weather conditions are anticipated during the
next few days, due to cold and wintry conditions.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory from midnight tonight to 6 PM MDT Sunday
     for WYZ101-102.

     Freezing Fog Advisory from midnight tonight to 6 PM MDT Sunday
     for WYZ116-117.

NE...Winter Weather Advisory from midnight tonight to 6 PM MDT Sunday
     for NEZ002-003-095-096.

&&

$$

SHORT TERM...REC
LONG TERM...MD
AVIATION...SF
FIRE WEATHER...REC



Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 638 PM CDT Sat Oct 30 2021

.SHORT TERM...
255 PM CDT

Through Sunday night...

The forecast point of emphasis for tonight is cloud cover, one
because it is the meteorological element of most challenge over
the next 12 hours, and two because there is heightened interest
due to possible viewing of the northern lights. Otherwise through
the rest of the weekend, Halloween looks breezy with gusts of 25
to near 30 mph throughout the daytime hours, and a chill in the
air that will be felt by evening (wind chills in the 30s). This
also leads into the high likelihood of a freeze for part of the
area Sunday night.

The stubborn low pressure responsible for the wet end to the
workweek is finally on its way east passing over the central
Appalachians this evening. The outer edges of its cyclonic flow
will lose its grip of the area this evening, with surface winds
turning light northwesterly. The solid cloud shield has its back
edge roughly near I-80 in Illinois and over the southern part of
the lake as of 230 PM. There are some broken clouds still in
stretches north of there into eastern Wisconsin. This cloud cover
generally is moving to the south-southeast slowly within the
cloud-bearing northerly flow.

A sampling of the latest aircraft soundings indicated a fairly
weak inversion strength of 1.5C over about a 3 kft depth, which
would point toward a good sign of clearing the more cellular
activity currently north of I-80 . However, high resolution
guidance indicates this inversion will strengthen a bit after dark
(which is not uncommon), and likely will slow the rate of
clearing north, so suspect some areas of clouds will remain north
of I-80 and east of I-39 in the hours just after sunset. Advection
of the clouds will then become the main mechanism for clearing
through the evening. High resolution guidance, including the mean
of several of those in the 12Z HREF, supports this shoving clouds
gradually south-southeast, with most of the broken clouds out of
the CWA sometime in the early overnight or so. However that may be
the time more clouds start to approach from the northwest, though
the trend today has been slower with that, more so arriving after
daybreak Sunday.

A quick-moving cold front extends from the Arrowhead of Minnesota
to the Central Plains this afternoon and will sweep through the
area Sunday morning. This will remain progressive as it is tied to
a quick-moving, deepening low across the Canadian prairie
provinces. Because of that, it will provide gusty winds. While
there will be cloud cover along and behind it for a few hours
during the morning (maybe even brief sprinkles) to limit mixing
some, there is a stout isallobaric component and good cold
advection just above the boundary layer (850 mb temperatures
dropping around 6C in 6-8 hours across northern IL). Forecast
profiles show some spread in guidance, but top of the channel
winds look to at least support 25 to 28 mph and could be a few
gusts over 30 mph. This includes right behind the front as well as
during the afternoon when the sky should trend more sunny.

All Hallows Eve after dark will see the winds gradually slacken
but with the clear sky the temperatures will be falling. The winds
do not look to become calm during the night, but enough cold air
is in place for 29 to 32 degree lows across much of interior
northern Illinois. This will be the first freeze as well as the
first mid 30s into the heart of the Chicago area.

MTF

&&

.LONG TERM...
255 PM CDT

Monday through Saturday...

The extended period continues to feature dry and cooler than
normal conditions for the upcoming workweek as well as the area`s
first widespread subfreezing temperatures.

A broad upper trough will sag south across the Midwest this week
with persistent surface northwest flow keeping temperatures cool
across the region. Sunday night into Monday an upper jet streak
will be located overhead and to our east northeast. An axis of
associated precipitation develops in the favorable right entrance
region for ascent in western Nebraska. This will drift east
southeast through the day. At this time it looks like any remnant
precipitation should remain south of the local area, but some very
light precip or virga could drift across areas well south of
Interstate-80. For now will maintain a dry forecast. Multiple
mid-level waves dance around the upper trough through the period,
but limited moisture in the lower levels should limit this to
mainly increased cloud cover.

As for temperatures, highs during the period will generally be in
the 40s each day. Overnight lows dip down into the lower to mid
30s downtown and along the lakeshore. Those in the suburbs and
south of Interstate-80 will likely see lows in the upper 20s and
lower 30s. Those in northern central Illinois, toward Rockford,
could see temps dip into the lower 20s Wednesday night.

Petr

&&

.AVIATION...
For the 00Z TAFs...

Aviation Forecast Concerns:

* Gusty northwest winds and a period of MVFR ceilings behind a
  cold front Sunday.

Surface low pressure continues to slowly fill well east of the
region across the northern Appalachians early this evening, while
gradient slowly weakens locally. Winds have remained northeast off
the lake across KORD/KMDW, but should shift light northwest this
evening as temps drop and weak land-breeze develops. Otherwise,
last vestiges of stratocu will linger early this evening with VFR
bases around 3500 feet before skies clear.

Turning attention to our northwest, a cold front was noted
stretching from weak low pressure over northern Ontario southwest
across MN and northwest IA this evening. This front will push
east-southeast overnight in association with a mid-level
disturbance moving into the upper Midwest. Strongest forcing for
ascent and deeper moisture will remain north of the area across
northern WI/Lake Superior and no significant precipitation is
expected with the frontal passage locally. However, a period of
MVFR ceilings will likely lag the front during the morning/midday
hours with ceilings initially around/just below 2000 feet. Cloud
bases should rise through the day however, scattering to VFR
during the afternoon hours.

Northwest winds will increase behind the front Sunday morning as
well, from a combination of steeping low level lapse rates in cold
advection and strong pressure rises downstream of sprawling 1030+
mb surface high across the Plains. Strongest winds (gusts
approaching 30 kts) look to be mid-morning into mid-day, easing
somewhat during the afternoon. Gusts should fall off quickly with
sunset Sunday evening under clear skies.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters
     until 7 PM Saturday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...9 AM Sunday to 9
     AM Monday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...9 AM Sunday to 10
     PM Sunday.

&&

$$

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Area Forecast Discussion

National Weather Service Los Angeles/Oxnard CA

 804 PM PDT Sat Oct 30 2021

.SYNOPSIS...30/753 PM.

Cooler conditions and increasing low cloud and fog will continue
through this weekend. The tail end of a weak system will bring a
threat of light rain to northern sections on Monday. After a dry
and warmer Tuesday and Wednesday, another weak system will yet
again bring a chance of light rain to northern sections.

&&

.SHORT TERM (SAT-TUE)...30/803 PM.

Weak upper level trough of low pressure pushing through northern
and central California tonight. This in combination with
strengthening onshore flow near the surface will result in
deepening of the marine layer. The LAX-Daggett gradient peaked
at +4.6 mb late this afternoon, as compared to near zero yesterday
afternoon. Latest ACARS data showing a marine layer depth around
1600 feet across the LA basin this evening. NAM model cross
section showing marine layer should deepen to around 2500 feet
by Sunday morning, resulting in low clouds and fog pushing into
most coastal/valley areas. The deepening of the marine layer
combined with light southerly flow in the boundary layer will
likely bring patchy drizzle late tonight into Sunday morning
across coastal/valley areas of LA/Ventura counties, which has been
added to the forecast with evening update.

A zonal flow pattern will continue to bring high clouds into the
forecast area at times tonight through Sunday. The deepening
marine layer and slight lowering of heights will result in some
cooling temperatures on Sunday across interior areas.

*** From previous discussion ***

Overall, 12Z models in good synoptic agreement through the short
term period. At upper levels, a trough will remain over the area
through Monday then a ridge will build on Tuesday. Near the
surface, onshore flow will continue through the period although it
will weaken somewhat on Tuesday.

Forecast-wise, nothing too exciting is anticipated through the
period. The tail end of a disturbance will zip across northern
sections of the forecast district on Monday. This will bring a
chance of some light rain to areas north of Point Conception.
Rainfall total, if any, will generally be less than 0.10 inches
although the Rocky Butte area could get a little more.

Outside of the chance of light rain north of Point Conception, the
main forecast `challenge" in the short term will be the marine
layer stratus/fog. Current sounding data indicates marine
inversion about 1000-1500 feet deep. Overnight, will expect some
slight deepening of the inversion. So with the continued onshore
flow, and a possible eddy tonight, stratus/fog should push further
inland to most coastal valleys locales overnight. The stratus
should clear decently on Sunday although beaches could remain on
the cloudy side. This sort of marine layer stratus pattern will
persist through Monday. However for Monday night and Tuesday, some
rising H5 heights and weaker onshore gradients should result in a
bit less inland extent. Other than the potential stratus/fog,
skies should remain partly to mostly cloudy through Tuesday with
the mostly cloudy conditions occurring on Monday.

As for temperatures, will expect conditions to remain rather cool
through Monday. Coastal/valley areas will be about 5-10 degrees
below normal while inland areas remain 2-4 degrees below normal.
On Tuesday, temperatures will rebound a few degrees with weaker
onshore flow, less marine influence and higher H5 heights and
thicknesses.

.LONG TERM (WED-SAT)...30/210 PM.

Overall, 12Z models continue to exhibit good synoptic agreement
through the long term period. At upper levels, ridge will remain
on the area on Wednesday then a weak trough will move across the
state on Thursday, but the ridge will build yet again Friday and
Saturday. Near the surface, weak onshore flow will prevail through
Thursday then some weak offshore looks possible Friday/Saturday.

For Wednesday, it will be a rather nice day. There will be some
night and morning low clouds/fog, but otherwise skies will likely
be mostly clear. Afternoon temperatures will remain comfy, just a
few degrees below normal.

On Thursday, the tail end of another weak system will sweep across
northern sections. This will bring some partly cloudy skies and a
slight chance of light rain to San Luis Obispo county. Elsewhere,
dry conditions with partly cloudy to mostly clear skies will be
expected.

For Friday and Saturday, temperatures will rebuild as the upper
level ridge develops and surface flow potentially becomes weakly
offshore. Overall, the pattern will likely result in temperatures
around seasonal normals for the weekend.

&&

.AVIATION...31/0014Z.

At 2355Z over KLAX, the marine layer depth was 1200 feet and
slowly rising. The top of the inversion was at 2200 feet and 19
degrees Celsius.

Moderate confidence for 00Z TAFs. The marine layer is slowly
deepening today and this trend will continue tonight for
coastal/valley areas of LA/Ventura counties, where there will be
a chance of drizzle during the late night and early morning hours.
Slow clearing of low clouds today will allow for quick return of
cigs late this afternoon into evening for coastal TAF`s, reaching
valley TAF sites after 06z. Mostly expecting IFR/MVFR cigs
tonight, with potential for LIFR cigs at times, mainly KBUR, KVNY,
KSMX, KSBP, KSBA, KOXR, and KCMA.

KLAX...Moderate confidence for 00Z TAF. Mostly expecting IFR/MVFR
cigs through early Sunday afternoon, with a chance of drizzle late
night and early morning hours. There is a 20 percent chance that
MVFR cigs will linger through the afternoon hours on Sunday.
Good confidence that there will be no easterly wind component
greater than 6 kts.

KBUR...Moderate confidence in 00Z TAF. VFR conditions expected
through this evening, with onset of LIFR/IFR cigs sometime between
06z-08z and lingering through Sunday morning. There is a chance of
drizzle overnight into early Sunday morning.

&&

.MARINE...30/800 PM.

Generally high confidence in the current forecast. Winds and seas
are expected to be below Small Craft Advisory (SCA) levels across
all coastal waters tonight through Wednesday. There is then a 50%
chance of SCA level wind gusts at times Thursday for much of the
outer waters, and for the east Santa Barbara Channel portion of
the inner waters.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK (TUE-SAT).

Gusty Sundowner winds will create driving hazards for Santa
Barbara County Wednesday Night through Friday Night.

&&

$$

PUBLIC...Gomberg/Thompson
AVIATION...Gomberg
MARINE...Sirard
SYNOPSIS...Gomberg

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