474
FOUS30 KWBC 231554
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1154 AM EDT Sun Jun 23 2024

Day 1
Valid 16Z Sun Jun 23 2024 - 12Z Mon Jun 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
NORTHEAST TO THE OHIO VALLEY, SOUTH TEXAS, AND THE SOUTHWEST...

...Northeast to Ohio Valley...
Models continue to show the eastward push of a well defined
surface low across far southeast Canada into far northern NY state
and northern New England. Strong isentropic lift ahead of this low
in the plume of anomalous PW values over the northeast have pushed
an initial round of moderate rains across Northern New England this
morning. This will then be followed by developing convection along
the associated cold front this afternoon/evening pushing across
the Northeast, Ohio Valley, and Mid-Atlantic. Continuous moderate
rainfall with rates peaking around an inch per hour are likely
across far northern Maine, but limited instability has decreased
the flash flood threat. The Slight Risk was dropped for this
reason, but isolated flooding concerns remain. For portions of
Upstate New York, Vermont, and New Hampshire, organized convection
in the form of at least a few supercells is likely due to MLCAPE
of 1500-2000 J/kg and curved hodographs. This could lead to brief
instances of intense rainfall rates above 2" per hour, but fast
storm motions should limit the flash flood threat somewhat. 12z
HREF neighborhood probabilities depict 30-45% chances for at least
2" in 6-hours (ending 00z tonight) across NH and VT. Simulated
radars from the latest hi res guidance suggest that the frontal
convection Sunday afternoon/evening extending into the Ohio Valley
will be fairly progressive, limiting the flash flood potential
elsewhere as well.

...Southwest...
Anomalous PW values forecast to remain in place for the upcoming
day 1 period from southern California into the Southwest, with
values as high 5 standard deviations above the mean across southern
Arizona and southwest California. Similar to what occurred on
Saturday, another round of scattered convection likely over much of
Arizona into portions of New Mexico and southern Utah. Models are
beginning to show some convective potential in southern California
also, with the marginal remaining across this region. There remains
low confidence in where any isolated runoff issues may occur, but
there are HREF neighborhood probabilities as high as 60-80% for 1"
amounts and 10-40% for 2" amounts in south-central Arizona
suggesting some isolated runoff issues possible. Placement
agreement is not good in the latest hi res with HREF EAS for 1"
mostly less than 10% and near zero for 2". The previous marginal
was expanded northward to include more of eastern Utah and western
Colorado given the similar setup compared to Saturday, CAMs
simulated reflectivity, and current satellite trends.


...Far South Texas...
Surface low pressure across the southwest Gulf of Mexico forecast
to push westward and inland into northeast Mexico day 1.
Persistent east southeasterly low level flow expected to the north
of the low in an axis of PW values 2-2.5". Models again are showing
the heaviest totals remaining south of the U.S./Mexican border
across northeast Mexico, with scattered convection again likely
into far South Texas on the north side of the qpf axes. HREF
neighborhood probabilities are high for 1 and 2"+ amounts across
far South Texas, but the EAS probabilities are highest for 1"+ and
2"+ amounts to the south of the U.S. border. Given no big changes
overall to the pattern, the marginal risk was maintained over far
South Texas for isolated runoff issues.

Snell/Oravec


Day 2
Valid 12Z Mon Jun 24 2024 - 12Z Tue Jun 25 2024


...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE UPPER MISSISSIPPI VALLEY, UPPER GREAT LAKES, SOUTHWEST AND
NORTHERN MAINE...

...Upper Mississippi Valley/Upper Great Lakes...
The surface frontal boundary oriented northwest to southeast from
the Northern Plains into the Mid Missouri Valley at the start of
day 2 will begin to push back to the northeast toward the Upper
Mississippi Valley/Upper Lakes region. Strengthening south
southwesterly low level flow into this front will raise PW values
to 1.5-2" along and to the northeast of the front. Shortwave energy
pushing eastward across the Northern Plains toward the Upper
Mississippi Valley will help strengthen isentropic lift over this
front and support for overrunning convection. There is not a lot of
agreement on placement of any heavy precip totals, but overall
some potential given the favorable MCS set up. There is potential
for heavy day 2 rains to overlap where recent heavy rains fell
across southern MN/northern IA and into northern IL. An upgrade to
slight may be necessary in later issuances is better model
agreement on placement occurs.


...Southwest...
No significant changes expected day 2 to the large scale pattern
across the Southwest. PW values will remain highly anomalous from
southern California into the Southwest with a continuation of weak
forcing under the upper ridge axis stretching from the
Southern/Central Plains into the Southwest. Models continue to show
potential for widely scattered diurnal convection, with continued
low confidence with locations of any isolated heavy totals and
isolated runoff issues. No changes made to the previous outlook
across this area.

...Northern Maine....
Surface low pressure moving through the Northeast day 1 will slow
in response to the closing off mid to upper level center across
northern NY State and northern New England day 2. A comma
head/deformation precip band likely to form to the northeast of
this closing off mid to upper level low, spreading potentially
heavy precip into portions of northern Maine Monday. HREF 12 hour
neighborhood probabilities ending 00Z Tue are high for 1 and 2"+
amounts over northwest Maine and extreme northern NH. This may
overlap the heavy totals from day 1, with a marginal risk added for
this area.

Oravec

Day 3
Valid 12Z Tue Jun 25 2024 - 12Z Wed Jun 26 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY/MID WEST...

In the wake of the warm front pressing northeastward across the
Upper Mississippi Valley/Upper Lakes region day 2, the associated
cold front will be dropping southeast from the Northern Plains into
the Upper Mississippi Valley/Upper Lakes region day 3. Additional
convection likely to develop in the axis of high PWs pooling along
the front that becomes more west to east oriented. Convection may
train in a west to east direction parallel to this front as the low
level flow will also be westerly along and ahead of the boundary.
There still is spread in model qpf details, but the multi model
ensemble qpf mean and latest NBM do show potential for heavy
amounts across areas of southern WI/eastern IA and northern IL that
have received heavy rainfall recently. Across these areas, a
slight risk is depicted. A broad marginal risk was maintained
surrounding this slight risk area to cover the current model qpf
spread.

...Southwest...
Similar to the day 2 period, no significant large scale changes
expected during day 3. The anomalous PW axis, values 3-4+ standard
deviations above the mean will persist from southern California,
into the Southwest and southern Great Basin in a region of
continued weak forcing under the building closed high over the
Southwest. Widespread scattered diurnal convection again possible
across these areas, with continued low confidence in any qpf
details. Changes to the previous issuance were to expand the
marginal risk area to match the day 2 marginal risk area for
consistency given no expected changes to the overall pattern.

Oravec


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt