Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/22/23
Area Forecast Discussion...UPDATED
National Weather Service Indianapolis IN
738 PM EST Thu Dec 21 2023
.Key Messages...
- Light rain expected Friday into Friday night
- Temperatures 10 to 20 degrees above normal through next Tuesday
- Widespread rainfall expected Christmas day into Tuesday
&&
.Forecast Update...
Issued at 738 PM EST Thu Dec 21 2023
Minor adjustments were made but the forecast is largely unchanged
with this update. Raised low temperatures given light southerly flow
and warm advection as well as increasing clouds limiting the
radiative component. Blended model starting point has had a cold
bias the last couple nights in this regime.
There maybe some virga, sprinkles and/or non-measurable amounts as
top-down moistening continues by the early morning hours. Low
probabilities of measurable amounts should be confined to the deeper
moist conveyor belt across Illinois into west-central into northern
Indiana. Further south and east, the dry sub-cloud layer should be
deep enough to limit more substantial precipitation until deeper
moistening occurs later Friday.
&&
.Short Term...(This evening through Friday)
Issued at 259 PM EST Thu Dec 21 2023
Quiet weather continues across Central Indiana this evening into
this tonight. Latest satellite imagery does show widespread mid and
high clouds pushing in from the west due to continued upper level
moisture advection ahead of weak system in the Plains. Latest IND
ACARS soundings verifies this, showing a saturation within the 500-
300mb layer and then another area of saturation in the 925-850mb
layer. Forecast soundings and BUFKIT time height cross section
profiles show a gradual saturation through the entire column tonight
as moisture advection increases at all levels ahead of the
approaching subtle trough and associated broad, weak surface low.
Increased cloud cover will limit cooling tonight, keeping lows well
above normal for this time of year in the mid 30s to low 40s. Expect
warmest areas in Western and Northern Indiana with thicker cloud
cover and stronger warm air advection and cooler areas further south
and east where clouds will take longer to fill in.
Precipitation slowly pushes in from the west on Friday. Best forcing
for ascent will remain closer to the mid level vort max and lower
level wave/low pushing northeast into Southern Wisconsin. Short term
guidance continues to depict widespread showers and rainfall ahead
of the system pushing northeast into Central Illinois by Friday
morning, then entering the northern half of Indiana by Friday
afternoon and evening. With the best forcing for ascent further
north, highest PoPs will be across the northern and western 1/3 of
the forecast area. Light rain chances and scattered sprinkles are
possible late morning though mid afternoon from Vincennes to
Indianapolis to Muncie and points north and westward. However, the
steadier and heavier rain does not move into the area until after
sunset. Terre Haute to Lafayette to Kokomo will likely see steadier
and heavier rain first being closer to the main system. Most of the
precipitation overspreads the rest of the area after the short term
forecast periods ends, so refer to the Long Term for more
information on rainfall chances into Saturday morning. QPF amounts
through around 7pm Friday will be light, with most locations picking
up less than a tenth of an inch. In fact, the southern and eastern
half of Indiana may remain dry much of the daytime period. North
Central Indiana around Lafayette and Kokomo may see 6 hour totals
from 18z-00z (1pm-7pm) closer to two tenths of an inch as rainfall
will begin a few hours earlier in those locations. Despite increased
chances for rain this weekend, these QPF amounts likely will not be
enough to make much of a difference regarding drought status.
Continued southerly flow and warm air advection will result in
temperatures remaining 10-15 degrees above normal with highs in the
low 50s and lows not reaching freezing for the foreseeable future.
&&
.Long Term...(Friday night through Thursday)
Issued at 259 PM EST Thu Dec 21 2023
Friday Night through Sunday...
A moderated pattern will continue through this weekend with the
CONUS sitting beneath a polar jet retracted well north into Canada
...with the only amplified upper pattern (later this weekend)
placing Indiana under a ridging/further WAA regime. The gradient
between broad, stalled surface high pressure over the eastern US and
strengthening low pressure over the western Plains will further
promote above normal readings amid a moist-advective pattern off of
the Gulf of Mexico.
Higher-chance yet lower-QPF light rainfall will accompany a weak
short wave floating eastward amid the upper zonal flow through
Friday night. Any rainfall over 0.10 inches would be restrained to
the upper Wabash Valley with best forcing focused towards
CHicagoland and points north. Can`t rule out a couple stray showers
through the remainder of the weekend...although the bigger story
will be dreary skies Saturday trending to partial clearing on
Christmas Eve under amplifying ridging ahead of the deepening Plains
system.
Lows in the 40s and highs in the 50s will be nearing 20 degrees
above central Indiana`s late December normals. Should ample sun
peak out for several hours during the day Sunday, maximums in the
mid-60s will be within reach south of Bloomington.
Christmas Day through Thursday...
Decent model agreement continues on upper flow progression over the
central US through at least the early week...which will oversee a
strengthening cut-of trough slowly progressing eastward from near
Missouri...with what may well be 12-24 hours of steadier rainfall
around the Christmas Day/evening periods. Latest guidance is
presenting a more-encouraging phasing of a fist-shaped vort max
punching up into Illinois over corresponding stacked surface low. An
almost roped-out ribbon of both Gulf and Atlantic moisture would
then train from the southeast into Indiana...with the local CWA
perhaps under the bullseye of locally 0.50-1.00 inches. Best
chances for appreciable rain would be southern and far western
zones, with the fetch`s path over Moderate to Severe Drought an ever-
present mitigating factor.
Moderate confidence in a staunch dry conveyor rolling into central
Indiana around the early Tuesday timeframe. Confidence, however,
then decreases with the parent trough`s departure speed thereafter.
Low-chances of light rain will linger as the stacked cut-off slowly
rolls over Indiana. Snow will still not be expected amid above
normal temperature, with morning lows still above freezing through
the midweek. Any attempt of far northern colder (seasonal) air
plunging to the Midwest by the end of the long term will be dubious
with a very amplified upper ridge extending into interior Canada.
&&
.Aviation...(00Z TAF Issuance)
Issued at 604 PM EST Thu Dec 21 2023
Impacts:
* Rain chances increase Friday
Discussion:
Lowering ceilings are expected ahead of the next weather system. VFR
conditions are likely through at least late Friday, though low
probabilities of MVFR ceilings will exist starting Friday evening.
Rain coverage will increase through the day. Forecast confidence is
high.
&&
.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
Update...BRB
Short Term...CM
Long Term...AGM
Aviation...BRB
Area Forecast Discussion
National Weather Service Salt Lake City UT
308 PM MST Thu Dec 21 2023
.SYNOPSIS...High pressure will remain across the region through
Friday. A low pressure system will impact southern Utah Friday
night through Saturday night before a cold front crosses northern
Utah Saturday. Unsettled conditions then persist into Sunday
especially across northern Utah`s mountains before high pressure
returns early next week. High pressure is forecast to persist at
least through midweek, bringing a return to inversion conditions.
&&
.SHORT TERM (Through 12Z/5AM Sunday)...Early Thursday afternoon
analysis shows a northwest to southeast oriented high pressure
ridge axis centered directly over Utah. This is providing dry,
mild and stable conditions across the region. Latest aircraft
soundings at SLC indicate the low-level inversion has largely
mixed out, with some semblance of an inversion lingering near/just
downwind of the Great Salt Lake where mechanical mixing is less
efficient. A broad swath of stratus remains entrenched from Castle
Country, through the Swell and southeast into the valleys of
extreme southwest Colorado. Latest fog satellite product indicates
this stratus is beginning to erode from the edges, but given the
building ridge and lowest sun angle of the year, it will likely be
difficult to erode this stratus entirely, perhaps not until
Friday when a thickening veil of clouds will be on the increase
in advance of an approaching area of low pressure.
Speaking of an area of low pressure, global models and ensemble
members are in reasonable agreement in showing a low tracking
across Arizona on Saturday before opening up and propagating
eastward through the Four Corners region. Meanwhile, an upstream
northern stream trough is forecast to steadily amplify as it
approaches the Northern Rockies and eventually across northern
Utah and southwest Wyoming on Saturday. The greatest amount of
forecast uncertainty revolves around the amount of strengthening
of this northern trough and also the degree of phasing between
this trough and the southern stream low/trough. The Canadian
ensemble camp (and many of its past few deterministic runs) along
with a few members of the GEFS have been favoring a much stronger
northern stream system that closes off over/near northern Utah
along with much more phasing between the northern and southern
stream entities. This solution would yield higher snow totals
across northern Utah, particularly the mountains. An examination
of the Canadian, EPS and GEFS means actually reveal a good deal of
agreement, with these ensemble system means showing separation
between these two systems before they become fully phased just to
our east while evolving into a closed low system in the lee of
the Rockies late Sunday/Monday. A stronger, less progressive
northern steam low/trough would result in a longer period of
productive orographic snowfall generation across favored portions
of northern Utah`s mountains through Sunday with associated higher
amounts.
Digging into some of the details, it does appear that
precipitation will first develop across southern Utah within a
region of diffluent flow on Saturday morning, with precipitation
developing as early as Saturday morning across southern Utah high
terrain favored in moist, southerly orographics. Precipitation
will then gradually overspread most of south-central and southeast
Utah as we head through Saturday afternoon and evening. A newer
feature represented in 21.12Z runs is the presence of an organized
deformation banded feature that is forecast to set up across
southeast Utah Saturday while lingering into Saturday night.
21.12Z CAMs all depict this feature as well, but with differences
on placement, which is not uncommon with such features. Such a
deformation band has the potential to yield locally higher
precipitation totals than what has been forecast. An examination
of QPF across this region reveals 0.10-0.35" represented by the
ensemble mean, while ensemble max totals exceed 1" across SE Utah,
mainly in the extreme southeast corner. While a low probability
outcome, we cannot completely discount it at this point. Heavier
precipitation rates may also dynamically drive down snow levels.
Currently, snow levels are forecast to remain largely above 7,000
feet elevation for the bulk of the event before falling down to
the 6,000-6,500 foot range on the tail end of the event late
Saturday. An examination of snow level spread does reveal a 25%
chance that snow levels could fall to as low as 6,300-6,500 feet
across extreme southeast Utah by Saturday afternoon, resulting in
accumulating snow across Fiftymile mountain and other nearby
terrain, where 1-4 inches of snow is most likely. High-end totals
across southern Utah mountains do approach and in some cases
exceed 6". There is also some uncertainty on how north
precipitation will spread, with just about half of ensemble
members bringing measurable precipitation as far north as Castle
Country. There has been a notable southward shift in the forecast
over the last 24 hours, however. All of these trends and sources
of uncertainty will bear monitoring as these systems approach.
Farther north, precipitation associated with the northern stream
wave is forecast to develop shortly before sunrise Saturday
morning across northern Utah mountains and around sunrise in
valleys. Across the Wasatch Front, precipitation is expected to
begin as rain or a rain/snow mix before transitioning briefly to
snow before ending. Ensemble mean snowfall amounts across the
Wasatch Front range from a trace to just under 1", while ensemble
max (10% probability outcome amounts) are in the 1-3 inch range.
These higher amounts are reflective of some of the stronger,
slower runs that remain within the solution space. Even though
this outcome is a low probability one, it cannot be completely
discounted just yet. As colder air filters into northern Utah
Saturday in a region of cold advection, sfc-700mb delta Ts to
briefly become sufficient to support lake effect, but moisture
quickly strips away from the dendritic growth zone, per forecast
soundings. As a result, the window for lake effect snow is narrow,
limited to a ~3-6hr window Saturday afternoon. Given marginal
ingredients anyway, probabilities remain under 10% for lake
effect. To view the low-end, high-end and latest forecast snowfall
totals visit weather.gov/slc/winter.
.LONG TERM (After 12Z/5AM Sunday)...As the mean trough exits
downstream across Colorado on Sunday, initially some lingering
moisture and steeper lapse rates may continue to support a few
isolated/scattered snow showers, especially over the mountains,
although with little additional snowfall expected at that point as
drier air will start to rapidly move in. High pressure will build
in quickly behind this exiting storm system, settling across
northern Utah Monday and Tuesday. As the high pressure start to
builds in early Sunday, a strong northerly pressure gradient and
cold air advection with northerly 700 mb winds will support
Washington County gap winds. Currently the multimodel internal
guidance suggests just sub- advisory to near advisory level wind
gusts of around 40-45 mph in the favored gap areas. Have
introduced these local effect winds to the forecast Sunday morning
to start highlighting this potential, and will have to continue
to monitor future forecast trends.
Otherwise, we will quickly revert to inversion conditions by
Wednesday as the ridge axis crests the region and much warmer 700 mb
temperatures surge northward across the region. There remains some
uncertainty on the exact amplitude of this warming aloft, with
ensemble solutions range from -5C to +5C at H7. However, even middle
of the road solutions of around 0C would support high temperatures
well into the 40s at Alta (and other mountain locations), while
valley locations might struggle to get into the low 40s. Made some
manual adjustments to the forecast to increase the warming trend in
the mountains by Wednesday, and slow the warming trend in the
valleys due to this inversion setup. There remains plenty of
uncertainty beyond Wednesday with respect to a shortwave trough
potentially ejecting over the top of the ridge Thursday. The faster,
and farther south solutions would support a weak cold front passage
through northern Utah that would cool the mountains, but potentially
break the valley inversion and result in slight warming/clearing.
However, there are some other ensemble solutions that are slower
and/or farther north with this wave. Thus by Thursday, kept the
forecast at the consensus forecast from the NBM due to the increased
uncertainty.
&&
.AVIATION...KSLC...Valley haze will linger, likely waffling between
VFR and MVFR VIS, with lower VIS (MVFR) dominating overnight,
roughly 03-16z. There is a chance that fog develops over the lake
overnight (20% chance), but it is unlikely to impact the terminal.
Northwesterly flow will transition to light and southeasterly around
02-03z. Winds may be light and variable overnight.
.REST OF UTAH AND SOUTHWEST WYOMING...Valley haze will linger in the
Salt Lake Valley, northern Wasatch Front, and Cache Valley,
producing VFR-MVFR VIS. Winds will remain fairly light and diurnally-
driven. Fog development is most likely at KLGU, with VCFG possible
after 03z. If fog reaches KLGU, IFR-LIFR conditions will prevail
(40% chance)...with the most likely timing around 12-17z. Fog is
less likely at KHCR (20% chance).
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&
$$
ADeSmet/Church/Cunningham
For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity