← Previous May 31, 2023 4:39 PM

FXUS21 KWNC 261801
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 26 2023

SYNOPSIS: The synoptic pattern during the week-2 period is characterized by 
widespread mid-level high pressure over Canada and weak mid-level low pressure 
along both coasts of North America. This results in warmer and relatively quiet 
weather for most of the Contiguous U.S. (CONUS). Enhanced chances for excessive 
heat in parts of the Midwest are anticipated to wane early week-2, but as the 
anomalous mid-level ridge in Canada drifts west, there will be slightly 
enhanced chances for a period of nominally-excessive heat in the Northwest 
CONUS. A  favorable environment for thunderstorms with heavy rainfall in the 
Plains and some adjacent portions of the Lower Mississippi Valley and Rockies 
should slowly relent by the middle of week-2, when moist low-level inflow 
eases. The potential for river flooding mostly related to the spring thaw 
continues for portions of the West CONUS, Alaska, and along portions of the 
James River in South Dakota and Souris River in North Dakota.


Slight risk for heavy precipitation for portions of the Plains and adjacent 
Rockies, Sat-Tue, Jun 3-6.

Slight risk for excessive heat for portions of the Upper Midwest, and the Ohio, 
Middle and Upper Mississippi Valleys, Sat, Jun 3.

Slight risk for excessive heat for parts of the Northwest Sat-Fri, Jun 3-9.

Possible flooding for portions of California, Nevada, Utah, Wyoming, 
southwestern Montana, and Idaho.

Possible flooding along portions of the James River in South Dakota.

Possible flooding along portions of the Souris River in North Dakota.

Possible flooding in parts of western Mainland Alaska and along the Yukon, 
Kuskokwim, Koyukuk, and Kobuk Rivers.

Rapid onset drought risk across portions of the Upper Midwest, and the Middle 
Mississippi and Ohio Valleys.



FOR SATURDAY JUNE 03 - FRIDAY JUNE 09: The week-2 synoptic picture as depicted 
by ensemble model solutions for 500-hPa height anomalies resembles an omega 
block, with weak troughing along both coasts of North America, slightly 
below-normal mid-level 500-hPa heights along the southern tier of the CONUS, 
and expansive mid-level ridging over much of Canada. This general configuration 
continues through the period, although the anomalous mid-level ridge centered 
over Canada is expected to slowly retrogress toward western Canada and weaken 
slightly as the period progresses. As a result, warm and generally quiet 
weather is predicted to prevail across most of the CONUS. One exception to this 
is over part of the Plains and adjacent portions of the Rockies and Lower 
Mississippi Valley, where increased low-level jet (LLJ) activity creates 
enhanced probabilities of heavy precipitation with periodic thunderstorms for 
the region until the middle of week-2. Rainfall will be enhanced at times by 
upsloping orographic winds near the mountains. Reforecast tools from the GEFS, 
ECMWF, and CMCE all indicate at least a 20% probability of exceeding the 85th 
climatological percentile and 0.75 to over 1.0 inch for 3-day accumulated 
precipitation, supporting a slight risk of heavy precipitation across the 
region, Jun 3-6. This slight risk area is again extended farther north for the 
second successive day based on raw output from the CMCE, ECMWF, and GEFS 
ensemble means.

Expansive mid-level ridging over much of central North America is likely to 
result in warmer than normal temperatures across the central and northern 
portions of the CONUS, closest to the maximum 500-hPa height anomalies 
associated with the broad mid-level ridge centered in Canada. All reforecast 
tools indicate at least a 20% chance of maximum temperature exceeding the 85th 
percentile during Jun 3 from the northwestern Ohio Valley through the Middle 
and Upper Mississippi Valleys and the southwestern Great Lakes Region. The 
ECMWF reforecast tool and to a lesser extent the CMCE counterpart favor a 
warmer solution with 30% to 50% probabilities of exceeding the 85th percentile, 
but GEFS-based tools are cooler and bring in significant uncertainty, 
precluding designating any areas with a moderate risk of excessive heat. Dew 
points and associated heat indices are not expected to be extremely elevated, 
but nonetheless there is the potential for temperatures to exceed 90 deg F for 
portions of the Upper Midwest for the first time this season. The past one to 
two months have been drier than normal in this region, which increases the odds 
for higher temperatures but limits the potential for unusually high dew points. 
Taking all of this into consideration, a slight risk of excessive heat is 
posted from western Indiana westward through northern Missouri and northward 
across southern Minnesota, including part of the Chicago metro area, Jun 3.

Ensembles overwhelmingly favor a slow retrogression and de-amplification of the 
expansive ridge over Canada, which should bring much above-normal temperatures 
to the northwestern CONUS throughout the period. There is still a lot of 
uncertainty regarding the potential for hazardous heat, but with positive 
500-hPa height anomalies over and north of the region throughout week-2, 
chances are sufficiently elevated to post a slight risk for excessive heat over 
the Northwest throughout the period (Jun 3-9), with the best chances near the 
middle of week-2. Probabilistic tools show lesser chances for impactful heat to 
the west of the Cascades, and the lower-level wind flow pattern is not 
particularly favorable to bring excessive heat west of the Cascades, so the 
hazard has been removed from this area. In contrast, probabilistic tools based 
on all three model ensembles show enhanced chances for temperatures topping 90 
deg. F in parts of central and southwestern Idaho, so the slight risk hazard 
has been extended into this region.

Drought concerns are forecast to increase across parts of the central and 
eastern Corn Belt and adjacent areas during week-2 due to the aforementioned 
warmth, the expectation of below-normal precipitation, and precipitation totals 
one to three inches below normal for the past 30 to 60 days. Soil moisture is 
currently below the 30th climatological percentile over many of these areas. 
Due to these factors, a rapid onset drought risk is highlighted across parts of 
the central and eastern Corn Belt, specifically in much of southern Iowa, 
northern Missouri, central Illinois and Indiana, and southern Ohio.

Snowmelt continues across the western CONUS after accumulating a very large and 
in some cases record-breaking snowpack, inducing river flooding especially 
across higher elevations for some areas in the West, where a few sites still 
report over 40 inches of snow water content. As snowmelt continues in parts of 
the Sierra Nevada, Great Basin, and Northern Rockies, flooding may continue to 
occur along some rivers and in areas adjacent to the higher elevations. 
Flooding is also expected to continue along the James River in South Dakota as 
river levels have been very slow to recede due to saturated ground conditions 
and relatively flat topography. A similar situation has developed along the 
Souris River in North Dakota, which should cause ongoing minor to moderate 
flooding to continue into week-2.

Weak mid-level flow across Alaska favors only a slight tilt in probabilities 
for above and below normal temperatures across parts of northern and southern 
Alaska, respectively. However, regardless of temperatures, rivers are expected 
to continue to rise across western Mainland Alaska due to ice jams, while 
increasing snowmelt across eastern Alaska is anticipated to continue the 
potential for river flooding through a large part of the Alaskan Interior. 
Other rivers of concern for river ice break up leading to the potential for 
possible flooding and ice jams are the Kuskokwim, Koyukuk, and Kobuk Rivers. 
Another factor which could impact conditions over parts of Alaska is the 
evolution of Super Typhoon Mawar, currently west of Guam in the Pacific Ocean 
with top winds near 175 mph. Mawar is now the strongest typhoon since 2021, and 
the fifth-strongest May typhoon on record. The typhoon is expected to continue 
westward for the next few days before turning to the northeast just east of 
Taiwan. After recurvature, the storm is anticipated to slowly become 
extratropical while continuing northeastward, but still remain a potent storm 
system. Some tools bring the remnants of Mawar close to Alaska during week-2, 
which would significantly impact some parts of the state. However, model 
guidance has been inconsistent regarding how far north the storm tracks, and 
today’s runs are less indicative of a potential threat to Alaska than 
yesterday, so there are no highlighted hazard risks associated with the 
remnants of the typhoon at this time.

FORECASTER: Rich Tinker