Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/31/20
Area Forecast Discussion
National Weather Service Grand Rapids MI
631 PM EST Wed Dec 30 2020
LATEST UPDATE...
Aviation
.SYNOPSIS...
Issued at 323 PM EST Wed Dec 30 2020
- Evening snow showers by Lake Michigan then cold overnight
- Hazardous travel conditions New Years Day
&&
.DISCUSSION...(This evening through next Wednesday)
Issued at 323 PM EST Wed Dec 30 2020
-- Evening snow showers by Lake Michigan then cold overnight --
No changes to the forecast. We will see a brief window of non-
accumulating snow showers early this evening mainly by the
lakeshore. Temperatures in the lower 20s Thursday morning will be
more than sufficient to develop ice on wet untreated surfaces. This
could impact the Thursday morning commute.
-- Hazardous travel conditions New Years Day --
Not much change to earlier expectations overall but did raise New
Years Day ice accumulations somewhat in agreement with WPC guidance.
Currently, the most plausible scenario for New Years Day is
predominantly freezing rain south of I-96 where accumulations might
reach or exceed a quarter of an inch in spots around I-94.
Immediately north of this region, we are looking at generally 1-2
inch snow accumulations, with perhaps a lot of that being sleet.
Models have been surprisingly consistent with this general scenario.
However,for reasons discussed below, there is potential for a far
more complex outcome in terms of rapid changes of precipitation type
and intensity. While this should be far from crippling, travel
likely will become quite hazardous.
Forecast reasoning: Powerful upper low off the Pacific Northwest
will serve as an upstream kicker for the cutoff low currently over
northwest Mexico. This cutoff low features 500 mb heights that are
very rare in terms of how low they are at such a southern latitude.
There is some indication that models are not initializing this
feature well, perhaps due to a combination of the low`s highly
anomalous nature and sub-optimal sampling of the atmosphere over NW
Mexico (due to a relative lack of aircraft measurements and POES
swaths). All of this suggests we could see an especially compact and
intense upper PV max approaching our area on New Years Day.
With such strong and relatively localized dynamical forcing
possible, we could see a wide variety of precipitation types and
intensities unfolding New Years Day into evening. Deterministic
models have recently shown a slight run to run cooling trend at
lower levels. The character of warm air aloft is also far from
certain. This has great bearing on precipitation type and tends to
feature a lot of forecast sensitivity to begin with, and that is
before introducing any possible dynamical cooling effects we might
see with the upper PV max.
&&
.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 626 PM EST Wed Dec 30 2020
Winds will remain gusty behind the passage of a cold front through
the late evening. Expect we will see gusts 23-28 knots persist
for several more hours. MVFR ceilings near 2000 feet will remain
locked in place under an inversion and as winds remain off Lake
Michigan well into Thursday. We should start to see some breaks
and a more consistent rise above 2000 feet tomorrow evening as
winds turn southwest off the lake and warmer air begins to move
into the area.
&&
.MARINE...
Issued at 323 PM EST Wed Dec 30 2020
No significant changes to the forecast. The late Thursday morning
expiration of the Small Craft Advisory still looks on track.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 10 AM EST Thursday for LMZ844>849.
&&
$$
SYNOPSIS...TJT
DISCUSSION...TJT
AVIATION...HLO
MARINE...TJT
Area Forecast Discussion
National Weather Service Phoenix AZ
425 PM MST Wed Dec 30 2020
.UPDATE...
Updated Aviation
&&
.SYNOPSIS...
Generally dry conditions and slightly below normal temperatures
are expected through the rest of the week. Low temperatures will
dip into the 30s for many lower desert locations through Sunday,
with Thursday most likely the coldest morning. A mostly dry
weather system will move through the region on Thursday leading to
a period of cloudy skies, but any light precipitation chances
will be confined to the Arizona high country. Breezy northerly
winds are likely along the Lower Colorado River Valley Friday
following the weather system. High pressure Friday into the
weekend will result in mostly clear skies before increasing
cloudiness into early next week as another weak weather system
approaches the region.
&&
.DISCUSSION...
The deep upper level low that brought light showers to the area
yesterday has dug abnormally far south into northern Mexico, leaving
our region in a subsident regime with much drier conditions today.
Surface dew point temperatures are roughly 20 degrees lower than
yesterday and the dry conditions are further reflected throughout
the atmospheric column as seen with aircraft soundings out of
Phoenix. A tightened low level gradient developed last night on the
backside of the upper low and the winds have mixed to the surface
following sunrise this morning leading to some breezy north to
northeast winds. Strongest winds (20-30 mph gusts) are seen along
the Lower Colorado River Valley.The gradient should loosen through
this afternoon with winds subsiding this afternoon.
Surface temperatures this afternoon will see the benefits of the
modest warming aloft and clear skies as highs are forecast to top
out in the 63-66 degree range. This warming will however not prevent
another chilly night as lows on Thursday morning are likely to be
the coldest of the week. Few high clouds may pass over the area from
late tonight, but winds will be much lighter and surface dew points
will be lower. Many outskirt/open desert locations will near or fall
below freezing with the common cold spots experiencing the lowest
morning temperatures. Localized spots within the Phoenix metro area
will even see freezing conditions, especially towards the far
southeast Valley.
A closed upper low, currently seen west of Vancouver, BC, will dig
quickly south as an open positively tilted wave along the west coast
and likely close off again over AZ by late-Thursday or early-Friday.
Overall, less moisture will be associated with this disturbance,
focused mostly in the mid-levels and downstream of the trough axis.
Deterministic and ensemble models show a deeper moisture flux from
the northwest is quickly cutoff by Thursday afternoon by the drier
air aloft with the trough. There should still be enough mid-level
moisture for some high elevation light snow showers over northern AZ
Thursday and perhaps some isolated virga over the lower deserts. The
increased cloudiness Thursday will likely keep high temperatures
closer to 60 degrees and Friday morning lows a few degrees warmer
than Thursday morning across the Arizona deserts. With the low
center expected to pass over south-central AZ, winds will likely
remain light across most of southern AZ, but a breezy northerly
gradient wind will develop across southeast CA and the Lower
Colorado River Valley. After the mostly dry system exits the region
on Friday, west-northwesterly dry upper level flow will take over
across the region through the weekend. This will keep high
temperatures through the weekend at or a slightly below normals,
while overnight lows mostly dip into the 30s across the lower
deserts.
For early next week, model ensembles are mostly agreeing on a more
active pattern setting up through the eastern Pacific into the
Western U.S., but so far the weather systems still look to mostly
pass by to our north. The most likely scenario for our region for
the first half of next week involves continued dry conditions and
near normal temperatures. Any precipitation chances if a system were
to pass near or through our region may remain focused over high
terrain areas with limited moisture taps. The pattern may eventually
allow for a deeper system at sometime late next week or into the
following weekend in which there should be better rain/snow chances.
&&
.AVIATION...Updated at 2325Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Assessing wind directions will be the main weather challenge through
Thursday evening as high cirrus cigs begin to develop overnight,
then lower into a 12-15K ft AGL layer Thursday afternoon. Light W/NW
winds this evening may be somewhat variable in nature given the weak
pressure gradient, but should settle into the typical overnight
easterly by mid/late evening. East winds should prevail much of
Thursday though confidence is moderate-good that directions will
attempt to become west by sunset, albeit may remain light and
variable for many hours. This could allow for a switch in runway
configuration if desired early Thursday evening.
Southeast California/Southwest Arizona including KIPL and KBLH:
No significant weather issues will exist through Thursday evening
with only occasional periods of mid/high cloud decks. North winds
will remain preferred though speeds should be somewhat weaker than
the past couple days.
&&
.FIRE WEATHER...
Friday through Tuesday:
Dry conditions are expected at least through the weekend with
temperatures remaining at or slightly below normal. Humidity levels
stay modest with minimum RHs across the deserts typically in the 15
to 25 percent range day to day. Many lower deserts will see humidity
climb into the 45 to 70 percent range each night. Elevated winds can
be expected over the western deserts on Friday as north winds
increase in excess of 25 mph. Much less wind is anticipated Saturday
into early next week. A weak weather system looks likely by next
Tuesday, but precipitation chances look quite low.
&&
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&
$$
DISCUSSION...Benedict/Kuhlman
AVIATION...MO
FIRE WEATHER...Kuhlman