Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 01/19/22

Area Forecast Discussion

National Weather Service Denver/Boulder CO

 833 PM MST Tue Jan 18 2022

.UPDATE...
Issued at 833 PM MST Tue Jan 18 2022

All the ingredients are in place for freezing drizzle early
Wednesday morning. As is nearly always the case in these events,
uncertainty is very high in timing, duration, and amounts. That`s
because minor fluctuations in temperature of the saturated layer,
and how moist the near surface layer can become, determine a
large part of how much FZDZ can occur and thus, how much ice to
expect on bridges, overpasses, sidewalks, and some roads (or most
roads if enough drizzle falls). The cold front responsible for
all of this is still up in Nebraska and east-central Wyoming. It
should arrive by midnight but saturation in the low levels will
take a few hours after the winds shift northeast. By 3 AM
saturation may be reached across the northeast plains, and by 5 AM
pretty much all of our area along the east slope foothills below
8,000 ft, the I-25 corridor, and all of the plains could start to
see drizzle due to saturation in the lowest several thousand feet,
shallow upslope flow, and the fact that the temperatures in the
saturated layer continue to only be around -8 degC through midday.
Thus, freezing drizzle could occur across the I-25 corridor from
4 or 5 AM through about noon. If that happens, and the
lift/moisture is enough to generate decent drizzle, significant
impacts to travel could occur for the morning commute. Confidence
in those impacts is very low, and focused on how much drizzle
might occur. Temperatures will be plenty cold for it to freeze to
elevated surfaces and even some roads and sidewalks. That is the
reason for the Winter Weather Advisory: There are enough
ingredients in place, and the ensuing impacts could be significant
if enough drizzle falls.

The HRRR has recently picked (or technically re-picked) up on the
FZDZ for our area, perhaps providing a tiny bit of increased confidence
that impacts could materialize. Normally even CAMs have a hard
time generating FZDZ because of the shallow nature and extreme
sensitivity to temp/moisture profiles in time and space. The
overall forecast from this afternoon hasn`t changed much at this
point. We will know much more about drizzle amounts a few hours
after FROPA late tonight via surface obs, spotters, and webcams.
Snow amounts for the afternoon/evening hours are much more
certain, as the signal remains for some QG lift to overspread the
drizzle layer, and seeder/feeder processes will quickly win out to
generate light snow across much of the area, but additional
travel impacts (or for areas that don`t see any drizzle) due to
snow are expected to be limited.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 348 PM MST Tue Jan 18 2022

Main concern for impacts will be the potential for areas of
freezing drizzle and light snow late tonight into Wednesday
extending from the plains into the urban corridor. A light glaze
is possible on roads and other hard surfaces which may impact the
AM commute. A Winter Weather Advisory is in effect for the plains
and foothills early Wednesday morning into Wednesday evening.

Cross-sections show the post-frontal airmass further cooling and
moistening overnight. Light shallow upslope and weak synoptic lift
in combination with a moist layer to the surface will support light
precipitation and low ceilings. This will start in the NE corner of
CO and gradually progress westward Wednesday morning. Sounding
profiles have been consistently signaling potential for freezing
drizzle. Temperature profiles in the moist layer hold above -10 C
limiting ice crystal growth along with a dry layer above. With below
freezing surface temperatures, freezing drizzle is a possibility. By
early Wednesday morning, freezing drizzle chances increase westward
into the urban corridor in time for the morning commute. A light
glaze is possible on roads, sidewalks, and elevated surfaces which
may impact the AM commute. Model soundings show the NE plains to
cool quickly through the morning, so development of flurries/light
snow become more likely. Chances for freezing drizzle decrease from
east to west Wednesday afternoon and chances for light snow increase
as temperatures profiles further cool. Snow accumulations are
expected to be minimal.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 348 PM MST Tue Jan 18 2022

Models still show a subtle shortwave sliding across our area
Wednesday night. It`s trending weaker and further southwest
though. There`s always been a question of how much that would
interact with the low level cold air, but there`s at least a
chance it could reinvigorate some light snow for a while
overnight. This is now looking more likely over the mountains and
southern part of our area. Otherwise, the low level forcing for
precip will be fading and dropping south of our area during the
evening. We`ll hang on to the Winter Weather Advisory for the
foothills and Palmer Divide areas until 8 PM in case there`s
enough snow to keep things slick, but we may just have very light
snow continuing.

Temperatures Thursday are a challenge. There will be clearing, but
there might still be some clouds in the morning. This will also
depend on how much snow there is. We compromised between the
various solutions and will keep temperatures in the 30s on the
plains. Clouds will spread back in Thursday night ahead of the
next shortwave. There`s getting to be pretty good agreement now on
the Friday system. We`re on the edge of the QG lift as the system
shears out to our west, and the low level wind and temperature
fields aren`t too exciting. But there will be a zone of low level
convergence, a little cooling aloft, and upslope into the
foothills that will produce light snow over the mountains and
there`s a pretty good threat of a few hours of light snow on the
plains as well. This will mainly be during the day, though upslope
areas west and south of Denver could hang on into the evening and
these areas should have the greatest snowfall. Like Wednesday`s
system, it doesn`t look like much precipitation, but the
placement/timing could still mess up the Friday evening rush hour
around Denver and into the mountains.

Behind the trough, we`ll have NNW flow over the weekend with a
bit more wind in the mountains. Saturday looks sunny, but we might
not have displaced the cold air from the plains much yet. There
should be more of this on Sunday, with warmer temperatures at
least near the foothills and possible across the plains as well.
The models are slowing with the development of the next upstream
trough (lift arriving Tuesday instead of Monday now), but there is
a weak front in northwest flow bringing just a little cooling and
probably more clouds for Monday. There will be another chance of
snow along with cooler temperatures when the next trough arrives.
For now, this is slated for Tuesday into Wednesday, but it looks
like another system that might shear off more to the southwest so
some degree of additional cooling is likely but the threat of snow
is more speculative.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 457 PM MST Tue Jan 18 2022

This is challenging forecast for Wednesday. The general
ingredients are well understood, but trying to pin down details
and the impacts to aviation operations will be very challenging.
At a high level, a cold front moves through a little before
midnight, with ENE winds shifting to E and going from 12-15 kt to
10 kts or so by 1-3 AM. IFR CIGs will be delayed until midnight
or an hour or two after midnight given significant dry air to
overcome. Eventually the lowest levels saturate but it remains dry
aloft, a classic set up for freezing drizzle. The freezing
drizzle will transition to snow at some point Wednesday afternoon
or early evening. Amounts of FZDZ and snow will be light, but
even a few hundredths of FZDZ can cause impacts to runways and
aircraft, especially if VIS drops to under 1 mile and the CIG
drops below 003, both of which could happen. The issue is it`s a
very low confidence scenario in terms of pinning down when the
FZDZ may occur along with the lowest CIG/VIS. Our best bet for now
is some time between 3 AM and 11 AM. Light snow is possible
throughout the afternoon but under an inch of accumulation is
expected, and IFR/ILS conditions should continue well past
midnight into Thursday morning though not as low as during the
Wednesday morning hours. If there is any good news to this
forecast it is that winds should not be much of an issue. Much of
the day Wednesday they should be out of the east at 10 kts or so.

VFR conditions should return around sunrise on Thursday morning and
winds will be light out of the south.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 2 AM to 5 PM MST Wednesday for
COZ042-044-048>051.

Winter Weather Advisory from 5 AM to 8 PM MST Wednesday for
COZ035-036-038>041-043-045>047.

&&

$$

UPDATE...Schlatter
SHORT TERM...Danielson/Mensch
LONG TERM...Gimmestad
AVIATION...Schlatter



Area Forecast Discussion

National Weather Service Memphis TN

 906 PM CST Tue Jan 18 2022

.DISCUSSION...00Z upper air analysis this evening indicates a
mid-level trough axis over portions of the Central and Southern
Plains this evening. 02Z surface analysis places a developing
area of low pressure in the Red River Valley of Texas while a
ridge of high pressure is centered over the Carolinas. In
addition, an occluded front is located from the Great Lakes region
back through the central Plains. Southerly winds on the back side
of high pressure is bringing some milder air up into the Lower
Mississippi Valley. As of 8 PM CST, temperatures across the Mid-
South range from the middle 40s to lower 50s.

00Z KLZK upper air sounding indicates a substantial amount of dry
air present across the region. Short term models including
Convective Allowing Models (CAMs) suggest rain chances should hold
off until around daybreak as the atmosphere will need to
sufficiently moisten up across the Mid-South. Forecast overall in
good shape for the rest of the night.

CJC

&&

.PREV DISCUSSION... /issued 529 PM CST Tue Jan 18 2022/

UPDATE...
See aviation discussion.

PREV DISCUSSION... /issued 338 PM CST Tue Jan 18 2022/

DISCUSSION...
One more mild (if rainy) day ahead for the Midsouth, before
Arctic air arrives.

Models have remained consistent with timing and strength of the
Arctic front, which will pass through the Midsouth late Wednesday
afternoon through the evening. Low level moisture will be
sufficient for widespread showers, developing ahead of weak
shortwave ejecting from the Red River valley. Marginal instability
(MUCAPE around 600 J/KG) will support thunder south of I-40, but
mainly across north MS Wednesday afternoon.

As the shortwave exits by 03Z Thursday / 9PM CST Wednesday, precip
coverage should diminish. NAM depicts isentropic downglide on the
290K surface by this time. But with frontal depth increasing, the
290K surface as a whole will be lifting at a good clip, likely
enough to generate light snow or flurries, following a brief rain-
freezing rain and sleet transition. In all, little to no
accumulation of ice or snow is forecast. Main impact would be
possible light glazing on elevated surfaces such as signs and
mailboxes. A quick look at the 18Z HRRR depicts Memphis being dry-
slotted, with no freezing or frozen precip after rain tapers off
by early evening. NBM likely holds onto precip chances too long
into the night Wednesday night, but have kept low PoPs for
consistency and likelihood of sensible, if not measurable wintry
precip into the early overnight.

As mentioned previously, the biggest weather impact -by far- will
be the much colder air flowing into the Midsouth. Models have
locked onto the strength of the cold air and this afternoon
forecast remains consistent with earlier forecasts. Dry weather
appears likely into the weekend, as subtropical moisture remains
along the gulf coast. A weak clipper-type northern branch will
likely bring a brief shot of snow to the Ohio Valley Sunday night.
Will need to watch this feature for possible track change if the
overall longwave trof over the eastern CONUS is deeper than
currently forecast.

Winter is here for a while, with no significant warm ups on the 7
to 10 day horizon.

PWB

&&

.AVIATION...

No major updates to the forecast. VFR conds will prevail through
08Z. Expect MVFR cigs to move into the region as a cold front
moves into the area. Light showers will begin to develop around by
sunrise and will increase in coverage through end of TAF period.
Expect conditions to deteriorate by mid- morning to IFR by 15-18Z.
In addition, a few TSRAs could develop near KMEM between 19-23Z
tomorrow as the front moves into the vicinity. Southerly winds of
10-12 KTS with higher gusts at times will occur until the front
moves through. Will continue with low level wind shear tonight as
well.


&&

.MEG WATCHES/WARNINGS/ADVISORIES...
AR...None.
MO...None.
MS...None.
TN...None.
&&

$$



Area Forecast Discussion

National Weather Service Pocatello ID

 233 PM MST Tue Jan 18 2022

.SHORT TERM...Tonight through Thursday night. Satellite imagery
shows broad area of low stratus and/or fog throughout the Snake
Plain and slowly creeping into adjacent valleys, with a secondary
area over Bear Lake. Inversions remain robust overnight, and
guidance has trended into not only holding on to larger areas of
light fog, but returns the dense fog to a significant portion of
the region 02z- 06z. Dense Fog Advisory from earlier was extended
through tonight into Wednesday morning, and expanded further north
into the Arco Desert, Upper Snake Plain, and Shoshone areas of the
Snake Plain. There is strong evidence that the scenario could
actually repeat Wednesday night into Thursday ahead of the
incoming shortwave. Which may be problematic moving into Thursday
afternoon. More on that momentarily. Guidance remains steadfast
in bringing light snowfall into the central mountains Thursday
morning, spreading southeast across the remainder of the forecast
area during the afternoon. The bulk of the snowfall should occur
from mid afternoon to late evening, then lingering in the
southeast highlands ahead of the trough axis descending through
East Idaho late Thursday night into Friday. Snow accumulations
range from under an inch across shadowed portions of the Snake
Plain to roughly 3-5 inches in some of the higher elevations of
the eastern highlands for Thursday and Thursday night.
Accumulations of 1-3" look very reasonable at this time for the
main travel corridors, but some of the ensemble probabilities are
pushing the 2-4" narrative. This is where the fog scenario ahead
of the trough may come into play. If there is not enough available
mixing ahead of the main trough axis, it is entirely plausible to
see the stratus linger, and then advancing mid-cloud provide some
additional seeder-feeder potential into the lower elevation
areas, which would start the snowfall earlier and bump the totals.
DMH


.LONG TERM...FRIDAY THROUGH NEXT TUESDAY...
Snow showers will continue Fri AM with the shortwave trough axis
roughly overhead, along with breezy conditions at higher elevations
across the Central Mntns as well as across the ern Magic Valley and
srn highlands west of I-15. Have nudged these areas a bit higher
than NBM wind guidance through Fri afternoon (closer to the
experimental NBM 4.1 which does a much better job with our winds),
although still well below advisory criteria. Snow showers should end
by late AM for most areas, but may linger into the afternoon east of
I-15, with additional daytime accumulations generally a dusting to
an inch. As the shortwave splits, both pieces of energy progress
south/east of our CWA allowing a Pacific ridge of high pressure to
exert greater dominance Sat into next week. Our forecast focus will
likely turn back to temp inversions and fog/low stratus potential. A
shortwave riding the flow between the ridge and departing trough may
brush Island Park and the WY border mntns with light snow showers
Mon/Tue, but confidence is low as this will depend on its exact
track. High temps remain stagnant each day ranging from the mid 20s
to mid 30s. Early indications are that the nearby ridge may hold
intact throughout next week, keeping the active storm track well
north of our area. - KSmith


&&

.AVIATION...
Widespread IFR fog/low stratus continues to blanket the ern Magic
Valley and Snake Plain corridor including KBYI, KPIH, and KIDA.
We`re just not seeing anything that is going to significant break
up/scour out this activity, so have nudged TAFs down into continued
IFR conditions all day with only modest improvement to vsbys. After
sunset, HRRR cig/vsby guidance, HREF RH and sky ensemble means, and
NBM guidance all reinforce fog potential and slightly lower cigs
once again as the sfc cools/saturates, and given trends today and a
lack of any significant disruptive factor, have leaned on these with
all 3 terminals potentially reaching airfield minimums yet again
between 04-07z/9pm-midnight and staying there through late Wed AM.
Overall confidence is moderate (lower on whether or not KDIJ will
join the party as currently forecast...if anything, there are some
hints that cigs/vsbys could deteriorate again an hour or two earlier
than currently forecast this eve. We will need to continue to
monitor closely and amend as needed. Winds remain fairly light
throughout the period with no precip expected. At KSUN, the diurnal
wind shift to SE is now advecting fringes of the fog/low stratus
shield into the terminal. Covering the worst impacts with a TEMPO as
satellite shows the edge right over the terminal with potential for
it to drift in and out. A return to VFR is likely around 03z/8pm
there as soon as winds shift to NW and both introduce slight
downslope drying and also physically push nearby activity back out
of the Wood River Valley. At least some cig/vis improvement is
likely Wed afternoon, but another impactful descent into madness is
possible Wed night. Our next chance of snow and associated
impacts arrives Thu afternoon. - KSmith

&&


.PIH WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 11 AM MST Wednesday for IDZ051>055-057.

&&

$$