973
FXUS63 KGID 210140
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
840 PM CDT Mon May 20 2024

...Hydrology Update...

.KEY MESSAGES...

- BY FAR the main issue is the likelihood for at least a few (if
  not several?) severe storms this evening and overnight,
  followed by one final round of strong to potentially severe
  storms mainly during the first half of Tuesday daytime
  (particularly 9 AM-1 PM time frame). Large hail, damaging wind
  the main hazards, but a tornado or two is not out of the
  question especially if any isolated supercells can get
  established near/after sunset.

- Although not as big of a concern overall compared to severe
  storm hazards noted above, there is at least "some" concern
  for rainfall being abundant enough/heavy enough in narrow
  corridors to promote localized flooding/flash flooding tonight
  into Tues daytime, and a Flash Flood Watch has been issued
  for most of our Nebraska coverage area (CWA). Please see
  separate HYDROLOGY section below for more details.

- Once any strong/possible severe storms vacate our CWA to the
  east late Tues AM-early afternoon, we`ll have a fairly quiet
  and mostly dry 48+ hours (especially Wed) with little to no
  severe storm threat.

- From especially Thursday night onward, our forecast is again
  "littered" with various/intermittent chances for
  rain/thunderstorms, as a very active upper air pattern
  continues. At least for now, there are no "obvious" severe
  storm threats during this time (and we are not outlooked by
  SPC), but early signs do point to at least a marginal severe
  environment returning already Thurs evening and/or overnight.

- Temp-wise: Nothing unusual at all for late-May, with highs on
  most days between the upper 60s to around 80, and lows on most
  nights mainly 40s-50s.

&&

.UPDATE...
Issued at 822 PM CDT Mon May 20 2024

Please see updated HYDROLOGY section below regarding an areal-
expansion of the previously-issued Flood Watch.

&&

.DISCUSSION...
Issued at 507 PM CDT Mon May 20 2024

-- 7-DAY FORECAST OVERVIEW/HIGHLIGHTS (heavily focused on these
 first 24 hours):

- CURRENT/RECENT WEATHER SCENE AS OF 430 PM:
As expected, the vast majority of our CWA (save for some limited
shower activity mainly over Webster County this morning) has
remained dry and storm-free today thus far. Under what has
become partly to mostly cloudy skies, high temps have or soon
will top out in the 74-82 range most areas (coolest north/west
and warmest south/east. In the mid-upper levels, the main
player(s) tonight will be a series of subtle waves embedded in
west-southwesterly flow aloft, downstream from the main wave
still out over the Desert Southwest that will track directly
overhead the first half of the day Tuesday and take any severe
storm/flooding out with it. At the surface, nearly our entire
CWA currently resides just north of a developing/somewhat
diffuse warm front stretched across northern KS, with breezes
most areas currently out of the east-northeast 5-15 MPH. Surface
dewpoints are seasonably-moist...mainly low-mid 60s.

- THIS EVENING-OVERNIGHT:
Most of our CWA should remain storm-free until around 7 PM.
However, after that, and especially after 9-10 PM, all bets are
off as scattered to numerous/widespread thunderstorms are
expected to blossom over mainly our Nebraska CWA (along and
north of the warm front) as weak upper forcing arrives from the
southwest, and the low level jet (evident at 850 millibars)
begins to ramp up. Closely leaning on higher-res models such as
HRRR, a series of individual storms (possibly supercellular)
along with clusters and perhaps broken, mainly west east
oriented lines of convection will traverse our area overnight,
with a sharp south edge likely somewhere between Highway 6 and
the KS border (very little activity expected in KS). The later
storms form, there is some question as to how truly surface-
based they might be, but at the very least the environment is
primed for a large hail and/or damaging wind threat, given
mixed-layer CAPE at least 1500-2500 J/kg and deep layer shear
40-50 KT. This is a somewhat concerning environment as it is,
and if any storms are able to remain at least semi-
discrete/supercellular and tap into increasing low-level shear
mainly near/after sunset, then at least a brief/transient
tornado threat could be on the table as well, not to mention
some hydro concerns (see separate section below).

- TUESDAY DAYTIME:
Following what could be a relative lull in activity around
sunrise, the main upper wave and associated surface cold front
will cross overhead by late-morning/early afternoon. As this
occurs, one final round of more north-south oriented strong to
severe storms is expected to cross much of our mainly Nebraska
CWA from west to east between 9 AM-1 PM, perhaps offering up
more potential for large hail/damaging winds and localized
flooding. Once this activity clears, our severe storm and
flooding threat should end, as the severe threat focuses farther
east of our CWA through the afternoon and evening. Behind the
front, it will be very breezy/somewhat windy on Tuesday, with
northwest winds sustained 15-25 MPH/gusts 25-35 MPH. High temps
will depend on "exact" frontal timing, cloud cover etc. but we
are calling for roughly a 15-degree gradient from mid 60s far
north/west to upper 70s far southeast.

- TUESDAY EVENING-OVERNIGHT:
Most areas will likely remain dry, but isolated to scattered
showers and maybe a few WEAK storms could affect some counties
mainly south of I-80 into KS through the night. Winds will die
down, and this will be one of the cooler nights coming up with
lows mainly 44-51 most places.

- WED DAYTIME-THURS DAYTIME:
This is mainly a DRY/storm-free period in between upper
disturbances. High temps low-mid 70s Wed and upper 70s-low 80s
Thurs as south winds increase.

- THURS NIGHT-SATURDAY:
Intermittent shower/thunderstorm chances return as various upper
waves move through again. We`ll have to watch Thursday
evening/overnight for MAYBE our next, at least limited severe
storm chance.

-SUNDAY-MONDAY (Memorial Day):
The parade of low-confidence rain chances/PoPs continue as both
the ECMWF/GFS bring another disturbance somewhere across the
Northern/Central Plains. Although still several days out, at
least an early glance suggests instability levels fairly muted
by late-May standards and thus a lower severe threat.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 724 PM CDT Mon May 20 2024

- General overview for KGRI/KEAR:
This is a very complex period, with at least a few (if not
several?) rounds of thunderstorms possible through the first 18
hours (some of which could be severe), along with the likelihood
for several hours of MVFR/IFR ceilings (mainly after 04-06Z),
and finally an increase in northwest winds with breezy to
moderately-windy conditions prevailing much of Tuesday daytime.

- Ceiling/visibility/precipitation details:
Right out of the gate, scattered thunderstorms are in the
general area, and off-and-on thunderstorm activity could easily
continue through the majority of these first 18 hours, with the
best chance for any possible, multiple-hour break most likely
focused between 09-15Z. Some of the storms during the period
could be severe, which IF they were to directly impact KGRI/KEAR
could be capable of producing hail of at least quarter
size/wind gusts to 50+KT. By roughly 18Z, all convection should
be vacated east of the area.

As for ceiling/visibility trends, obviously visibility be
reduced at times during periods of heavier rain. As for ceiling,
the most pronounced period of MVFR/IFR is expected to focus
04-06Z through 18Z, with MVFR perhaps hanging on until very
late in the period closer to 22-23Z.

- Winds:
Aside from any direct thunderstorm-related influences (which are
actually fairly likely at times and could greatly increase
speeds and/or result in erratic directions), the basic
expectation is for: 1) winds generally around 8-13KT from the
east-northeast this evening...2) gradually switching to more
northerly at similar speeds late tonight-early Tues AM...3)
picking up out of the northwest Tuesday daytime commonly
sustained 16-22KT/gusts 25-32KT.

&&

.HYDROLOGY...
Issued at 839 PM CDT Mon May 20 2024

-- Latest comments regarding the Flood Watch valid through 1 PM
 Tuesday that was recently included to include 6 more counties
 in our Nebraska CWA:

- Within the last hour, the Flood Watch was expanded to include
  6 more counties along and south of the Highway 6
  corridor...mainly in response to last few HRRR runs shifting
  axis of overall-heaviest rain south a bit from runs of 6 to
  especially 12 hours ago.

- Still believe the MAJORITY of our Nebraska counties sees
  widespread amounts no more than 1-3" (and with little to no
  flooding issues given that most areas should be able to take
  on at least 1-2" given lack of very recent heavy rainfall).
  However, with potential for at least a narrow swath of perhaps
  3-5" still in play, felt the continuation/expansion of the
  Watch was warranted.

- There is still uncertainty in the EXACT placement of potential
  amounts of 3+", but given that it could be almost anywhere
  OTHER THAN probably Webster/Nuckolls/Thayer counties (least
  favored of all to see higher-end rainfall), wanted to be more
  generous with the Watch area.

&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...Flood Watch through Tuesday afternoon for NEZ039>041-046>049-
     060>064-072>077-082>084.
KS...None.

&&

$$

UPDATE...Pfannkuch
DISCUSSION...Pfannkuch
AVIATION...Pfannkuch/Thies
HYDROLOGY...Pfannkuch