Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 242344
AFDOUN

Area Forecast Discussion
National Weather Service Norman OK
644 PM CDT Sat Oct 24 2020

...New AVIATION...

.SHORT TERM...
(This evening through Tonight)
Issued at 342 PM CDT Sat Oct 24 2020

Much of the focus is in the long term portion of the forecast.
Current satellite and surface observations show clearing
conditions and light winds that have been trending southeasterly
throughout the day. This will result in a milder night with mid
30s northwest to mid 40s south for lows.

BRB

&&

.LONG TERM...
(Sunday through next Friday)
Issued at 342 PM CDT Sat Oct 24 2020

The long term portion of the forecast features anomalous cold
juxtaposed with a few rounds of mainly light precipitation. Impacts
from wintry precipitation appear possible Monday morning through at
least the first half of Tuesday, but there are some caveats and
failure modes. Uncertainty remains fairly high, especially with the
position of the freezing line. Our forecast continues to convey the
most likely outcome of a prolonged intermittent light freezing
rain and minor accumulations along and northwest of the I-44
corridor where greatest confidence in sub-freezing temperatures
exists. The greatest time period for impactful wintry
precipitation including some icing appears to be Monday night
through the first half of Tuesday at this time. Forecast changes
were mostly minor but include the following:

1) Increase of post-frontal drizzle Sunday and minor refinement of
freezing line and where light icing may begin as early as Sunday
evening in northwest Oklahoma.

2) Restrict where snow is emphasized to a smaller area of northwest
Oklahoma due to more dominant freezing rain and sleet precipitation
type per forecast soundings.

3) Tried to emphasize higher precipitation probabilities the first
half of Monday, and again Tuesday morning with two distinct forcing
mechanisms, while communicating lighter mostly sub-measurable
amounts of drizzle in between.

Please see below for a detailed analysis including best and
worst case scenarios.

Our most impactful freezing rain events generally occur when
sufficient forcing for ascent occurs for high precipitation amounts
in a favorable wind and thermal setup to maximize ice accretion
efficiency. On the synoptic scale this usually this involves a
somewhat out of phase system with a leading northern stream
shortwave trough in the northern stream that dislodges a cold cP air
mass and sends it southward through the Plains, and a trailing
southern stream wave that provides forcing for ascent atop a shallow
cold air mass. This setup likely falls short on the magnitude of
cold air and the intensity and duration of precipitation for major
impacts, though it seems increasingly likely to be enough for minor
impacts.

The ECMWF/EPS and NAM are quicker with closing off the westernmost
end of the trough and tracking it further west, thus developing a
more split/out of phase system. This is probably a major contributor
to less significant cold surge, or at least is limiting its eastward
extent. So, a fairly substantial gradient may be present across our
area making specific placement of impactful wintry precipitation
difficult. Low-level streamlines show confluence behind the cold
front tomorrow, and forecast soundings have sufficient low-level
saturation for drizzle. Late in the night continued cold advection
would support sub-freezing temperatures across northwest Oklahoma
and some light freezing drizzle.

Models have consistently showing a PV anomaly enhance ascent and
increase deep saturation resulting in more than just drizzle, and
generally increased rainfall rates through the first half of Monday,
especially across the southeast two-thirds of the area where 0.25-
0.50 inch QPF will be common. This is mostly misaligned with sub-
freezing temperatures, fortunately. Continued cold advection Monday
will offset any diurnal heating and result in steady or slowly
falling temperatures during the day, with decreasing temperatures
overnight into early Tuesday. Upstream cold air mass is concerning,
and it`s possible that models will trend toward a stronger density
current with colder/shallower air mass with time, which would bring
sub-freezing temperatures further southeast. Right now, our concerns
across the southeast third of the area are minimal, but again we`ll
need to watch for trends toward colder temperatures which would
result in a broader area of freezing rain.

From a forcing/precip amounts standpoint, as the closed low from the
west moves east, a broad area of deep isentropic ascent and
moistening will result in increasing precipitation late Monday
night. Early Tuesday is the most concerning period with some
potential for an impactful ice event. At least some glaze seems
increasingly likely, but impacts to power cannot be ruled out
somewhere given 0.10-0.25" ice amounts, most likely in a band near
and just northwest of I-44, generally from near Enid, to El Reno to
Hobart, to Altus. We hesitate to get that specific now as a more
robust cold surge and shift in QPF maxima may result in slightly
eastward for this ice accumulation maxima. An additional note with
regards to impacts, leaves are still on the trees and this could
augment typical impacts expected with the amounts we`re currently
forecasting. Furthermore, from a Freezing Rain Accumulation Model
(FRAM) standpoint; temperatures, wind, and precipitation rate
could yield optimal ice accretion and high ice:liquid ratios.

One other uncertainty is the exact depth of cold layer,
particularly across the northwest. There may be a band of
predominately sleet which would obviously limit icing
accumulation. These situations typically have a fairly sharp warm
nose (often sharper than models indicate) which should generally
limit snow, although we did include at least low snow
probabilities for far northwest Oklahoma. As of now, snow/sleet
accumulations appear minor, but we`ll need to watch thermal
profiles closely to refine precip types and ultimately ice/snow
amounts.

The good news is this isn`t the setup where strong frotongenetical
forcing and heavy banded precipitation is expected, unlikely last
year on this exact date if you recall. October 24th of last year, a
heavy mesoscale snow band resulted in locally 13" snow accumulation
in northwest Oklahoma. Just goes to show that although anomalous,
impactful wintry precipitation can occur this early in the season.

Best case scenario: Rainfall amounts are less and cold air isn`t as
extensive resulting in less icing, but even in this best case
scenario somewhere across the northwest half of Oklahoma is likely
to see a light glaze of ice.

Reasonable worst case scenario: Greater magnitude of cold air and
quicker, with more QPF and optimal liquid:ice ratio to result in
localized impactful ice amounts to trees and powerlines. If this
scenario occurred it would likely be near and just northwest of
I- 44. Although greater overall coverage of at least some icing
could be expected, possibly as far southeast as Lake Texoma and
Coalgate.

We set a record cold-maximum both Monday and Tuesday.

Record cold-maximum for Monday Oct 26:
OKC 42
LAW 48
SPS 48

Record cold-maximum for Tuesday Oct 27:
OKC 43
LAW 46
SPS 46

Forecast uncertainty grows late Tuesday through the rest of the
week. Warm advection should increase our temperatures late Tuesday
to limit wintry precipitation concerns, although some lingering
wintry precip may occur in northwest Oklahoma through early
Wednesday morning. Deterministic and ensemble guidance is in
decent agreement with closed low track, but minor variations in
the track and speed have significant implications for
precipitation amounts. It`s likely that the majority of
precipitation during this period would occur from
deformation/TROWAL type setup, with some mesoscale enhancement
possible. GEFS and EPS ensemble data gives us as much as 3-3.5
inches total QPF over the 7-day period, much of which occurs from
aforementioned banding which could shift some. These type
scenarios typically fall on the low-predictability end of the
spectrum of forecast confidence. Fortunately, confidence is high
that temperatures will be warm enough to not have to deal with
further winter weather precip impacts.

Finally, we will remove a few counties from the Freeze Watch for
Sunday night/Monday morning, were a hard freeze occurred early this
morning.

BRB

&&

.AVIATION...
(00Z TAFS)
Issued at 639 PM CDT Sat Oct 24 2020

VFR conditions are expected through much of the evening before
lower clouds (widespread MVFR and locally IFR ceilings) begin to
spread in from the north Sunday morning.

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  41  56  34  35 /   0  20  70  80
Hobart OK         42  58  30  32 /   0  10  60  70
Wichita Falls TX  46  71  37  39 /   0   0  40  70
Gage OK           36  44  21  26 /  10  30  70  70
Ponca City OK     39  51  31  33 /   0  20  70  80
Durant OK         44  69  45  48 /   0  10  60  80

&&

.OUN WATCHES/WARNINGS/ADVISORIES...
OK...Freeze Watch from Sunday evening through Monday morning for
     OKZ006>008-011>013-017>020-022>030-033>039-044.

TX...Freeze Watch from Sunday evening through Monday morning for
     TXZ083>088.

&&

$$

SHORT TERM...11
LONG TERM....12
AVIATION...26


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