214
FXUS63 KGLD 210925
AFDGLD

Area Forecast Discussion
National Weather Service Goodland KS
325 AM MDT Sat Sep 21 2024

.KEY MESSAGES...

- Showers and thunderstorms will develop over majority of the
  Tri-State area this weekend. Both severe weather and flooding
  chances are very low (<5%).

- Low temperatures in the mid to upper 30s are forecast for
  eastern Colorado by Monday morning. There is a concern for fog
  formation Monday morning, too.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 320 AM MDT Sat Sep 21 2024

This morning, the low pressure system that we have been
monitoring for the past week will be near the Four Corners region
and moving east-northeast. Over the past few days, guidance has been
very gradually slowing this low, which is delaying the start of the
precipitation. Another notable change over the previous 36-48 hours
is that the low looks to be taking a more easterly path than
previously thought. These two changes have broadened out the low,
making it weaker, and has shifted it`s main path farther south,
likely over the south-central CWA when it finally crosses. The
broadening will push a the cold front into the area early this
morning, likely crossing the entire CWA by 18Z. With this cold
front, northerly winds sustained around 20-25 kts and gusts around
35 kts are expected this morning and mid-day. These winds will cause
temperatures to remain cool, likely hitting maximum temperatures
before noon. Temperatures today in the northwestern CWA will be in
the low 70s, but the southeastern CWA will still warm into the low
80s before the cold front passes.

This cold front coming earlier than previously thought, and the more
southern track, has lowered total QPF for the northwestern half of
the CWA. Guidance is showing the 500mb shortwave that will set off
the precipitation will start forming showers and storms south of the
CWA by 16-18Z. Over the following hours, this precipitation will
move northeastward while new showers will form west of the already
existing precipitation. No later than 21Z, we expect to see
precipitation in the CWA. The best coverage of rain for will start
around 0Z. From 0Z through 12Z, all of the CWA will have 50-75+
PoPs. After 12Z, PoPs taper off from the southwest as drier air
moves in and pressure rises occur following the low. By 0Z Monday,
all of the precipitation will have moved out of the area. For more
information about the precipitation, see the hydrology discussion
below. Severe convective weather seems extremely unlikely (<2%
chance) due to a lack of instability.

Temperatures overnight tonight will be a balancing act between the
CAA and the clouds insulating us. Currently we expect lows to cool
into the mid 40s in the west to low 50s in the east. However, if the
northerly winds are stronger, expect cooler temperatures (~5 degrees
cooler), but if the winds/CAA weaken, expect lows near 50 for the
entire Tri-State area. Sunday`s high temperatures will be the
coolest we`ve seen so far this season. The clouds and northerly
winds will prevent majority of the day time heating. Temperatures
are expected to remain in the low to mid 50s Sunday.

As the low moves out of the area Sunday afternoon/evening, winds
will weaken and the sky will clear out. This is creating an
interesting setup overnight Sunday where two things could happen, or
a mixture of both. Potential one is patchy to widespread fog; with
clear skies, light winds, and recent precipitation, the environment
is primed for fog formation, with a catch. The light winds that will
exist will be westerly, downslope winds, which will work to prevent
fog formation. Most likely scenario is valleys and locations
sheltered from the light breezes will see fog form, but if the winds
drop to 0, widespread fog could easily form. Potential two is frost
formation, mainly in eastern Colorado. The same conditions that make
fog possible/likely Sunday (minus the moisture) could allow for
temperatures to drop into the mid 30s, allowing for light, patchy
frost to form. The big unknown, and biggest limiting factor, for the
frost potential is what the dew points will do Sunday
evening/overnight. If dew points remain in the low 40s in eastern
Colorado Sunday evening/night, the potential for frost is near 0.
However, if the dew points are in the mid 30s, patchy frost becomes
more likely. Confidence for "potential one," the fog, occurring is
around 30% and could impact majority of the CWA. Confidence for
"potential two," the frost, occurring is around 25%, but looks to be
limited to our Colorado counties. As mentioned before, there is a
chance (~20%) that both could occur; we could see fog in the eastern
CWA and frost in the western CWA. We will be monitoring this closely
over the next 36 hour.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 245 AM MDT Sat Sep 21 2024

Monday: Guidance suggests that troughing/cyclonic flow aloft
will persist over the Central Plains.. on the eastern fringe of
an amplifying upper level ridge along the Pacific Coast. Clear
skies in the wake of the departing wave may eventually give way
to increasing cloud cover late Mon-Mon night.. via cyclonic
shear vorticity on the eastern periphery of a strong (~100-115
knot) northerly upper level jet extending southward from Alberta
and Saskatchewan to the central Rockies. Sunny skies during the
day will foster a warming trend. Expect temperatures warmer
than Sunday (albeit still cooler than average).. ranging from
the mid 60s (east) to mid 70s (west).

Tuesday-Friday: Below average forecast confidence. Forecast
confidence typically decreases with range -- `below average` in
this context is intended to communicate a further reduction in
confidence associated with /inherent to/ increased pattern
complexity -- e.g. patterns with cut-off waves, multibranched
wave interactions and potentially significant forms of
constructive/destructive interference or feedback.

Solutions from current operational runs of the GFS/ECMWF
continue to diverge on Tuesday.. albeit less-so than yesterday.
Both models suggest that [1] the aforementioned ridge will shift
eastward across the Intermountain West (Tue), Rockies (Wed),
Central/Northern Plains (Thu) and Mississippi River Valley
(Fri).. [2] that some degree of northern and southern stream jet
phasing (or interaction) within a complex split flow regime on
the eastern periphery of the advancing ridge will foster the
development of a closed low and [3] that said low will be
subsumed by /trapped within, cut-off beneath/ the ridge. The
ECMWF suggests that upper low development and subsumption will
occur over the Central-Southern Plains.. the GFS suggests it
will occur over the OH Valley. Steering the forecast toward
climatology appears to be the most prudent course of action at
this time.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1036 PM MDT Fri Sep 20 2024

VFR expected to prevail at both KGLD and KMCK through the TAF
period. A cold front will shift winds to the northwest then
northeast early Saturday morning, gusty through the afternoon.
Some showers may begin to move into the area towards the end of
the TAF period Saturday night, but with minimal, if any,
impacts.

&&

.HYDROLOGY...
Issued at 320 AM MDT Sat Sep 21 2024

The 850mb LLJ has been moving moisture into the Great Plains
overnight and will continue into Sunday, following the low as it
moves east out of the area. Due to the more southerly path the low
is expected to take and the early cold front, most of this moisture
will not directly be moved into the CWA. The moisture will instead
be moved into the area by the low as wrap-around moisture, as seen
on 315K maps.

Precipitation looks to start in the eastern CWA late Saturday
morning/early afternoon, intensifying Saturday evening, and begin
weakening Sunday morning/mid-day before moving out of the area by
Sunday evening. Between 0Z-12Z Sunday, nearly the entire Tri-State
area will see at least a few tenths of rain with the potential (20-
25%) of seeing 1+ inches in that timeframe. Concerns for flash
flooding are low (<5%) with the potential for areal or river
flooding being slightly higher at around 20%.

Throughout the entire event, locations along and southeast of a line
from Wallace, KS to Hill City, KS can expect to see .9-1.7 inches of
rain, with pockets of 2.5+ inches being possible. Locations west and
north of that line, but south of HWY 36 will receive around 0.4-1
inch of rain, with some pockets up to 1.75 inches. Areas north of
HWY 36 will see the least amount of rain, generally ranging from
around 0.25-.8 inches.

&&

.GLD WATCHES/WARNINGS/ADVISORIES...
KS...None.
CO...None.
NE...None.

&&

$$

SHORT TERM...CA
LONG TERM...Vincent
AVIATION...024
HYDROLOGY...CA