MESOSCALE PRECIPITATION DISCUSSION 0673
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
255 PM EDT SAT AUG 12 2017

AREAS AFFECTED...SOUTH CENTRAL ND...NORTHEAST SD...

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 121855Z - 130000Z

SUMMARY...NEARLY STATIONARY CELLS CURRENTLY POSE FLASH FLOODING
RISK BEFORE MAIN SHORTWAVE/DEEPER CONVECTION DEVELOPS AND SHIFTS
EAST.

DISCUSSION...EXPERIMENTAL GOES-16 WV SUITE DEPICTS A ROBUST
SYNOPTIC WAVE OVER W ND...WITH STRONG UNDERCUTTING JET STREAK
EXITING SE MT ATTM.  THIS HAS ALLOWED FOR A FAIRLY SHARP SHORTWAVE
RIDGE TO DEVELOP JUST EAST OF 100W FROM CENTRAL ND TO CENTRAL SD
WITH STRONG DIFFLUENCE INDICATIVE OF CIRRUS PATTERN ACROSS THE
EASTERN THIRD OF THE DAKOTAS.  AT THE SURFACE A WAVE OVER CENTRAL
SD IS CORRALLING LOW LEVEL FLOW/DEEPEST CHANNEL OF MOISTURE FROM
THE DUE SOUTH ACROSS CENTRAL SD/N NEB...WHILE A WEAK SURFACE WARM
FRONT/COOL CONVEYOR EXTENDS EASTWARD ACROSS EASTERN SD...BRINGING
ESE AND SE FLOW FROM CENTRAL MO/BIG SOUIX RIVER VALLEYS.   THE
CONFLUENCE SUPPORTS INCREASED MOISTURE FLUX AND TPWS OVER 1.25"
THOUGH GOES-16 TPW SUGGESTS SLIGHTLY HIGHER MAGNITUDES MAY BE
AVAILABLE.

AS SUCH CONVECTION HAS DEVELOPED ALONG THE DEFORMATION/WARM
FRONTAL ZONE WHICH GIVEN THE HIGH DIFFLUENCE/MID-LEVEL RIDGING
HAVE VERY WEAK STEERING/PROPAGATION VECTORS TO ALLOW FOR SLOWED
FORWARD PROGRESS FOR RATES OF 1.5"/HR TO LINGER FOR 1-2 HOURS
BEFORE SLIDING EAST.  BACKYARD WX STATION MESONET IN NAPOLEON AND
GACKLE IN LOGAN COUNTY HAVE SUPPORTED 2.45"/3HRS AND 1.52"/1.5HRS
RESPECTFULLY.  AS SUCH OU FLASH/CREST UNIT UNIT STREAM FLOWS ARE
GREATER THAN 2-4 CM/KM2 AND 3HR QPE/FFG VALUES ARE GREATER THAN
1.0".

FURTHER SOUTH...UPSTREAM DPVA/HEIGHT FALLS SUPPORT BACKBUILDING
WITH EVEN HIGHER MOISTURE FLUX ACROSS HYDE COUNTY IN E CENTRAL
SD...ALOS POSING FLASH FLOOD AS QPE/FFG RATES ARE NEAR/AT 1.0"
THERE AS WELL.

LONGER TERM...AS STRONG SHORTWAVE/JET STREAK APPROACHES OVER THE
NEXT HOUR OR SO...STRONGER MST CONVERGENCE SHOULD ALLOW FOR
INCREASED EXPANSION DEEPER CONVECTIVE COMPLEX BETWEEN THE TWO
ONGOING AREAS. RECENT RAP ANALYSIS SUGGESTS INCREASED INSTABILITY
(SBCAPES 2000+ J/KG) WITHIN THIS AREA WAS ABLE TO BUILD UP GIVEN
LOCALIZED STRONGER LOW LEVEL CAP...WHICH CONTINUES TO STEADILY
ERODE.  THE STRONG FORCING/INSTABILITY WILL SUPPORT DEEPER
CONVECTION WITH POTENTIALLY HIGHER RATES IN EXCESS OF
1.5-1.75"/HR.  THOUGH WITH THE STRONGER FORCING ALSO COMES ERODING
SHORTWAVE RIDGING/INCREASING STEERING/FORWARD PROPAGATION LIMITING
SOME TOTALS YET...1-3" VALUES ARE STILL PROBABLE MAINTAINING SOME
FLASH FLOOD RISK.

INDIVIDUAL CELLS THAT CAN REMAIN NEAR THE INFLECTION OF THE
DEFORMATION ZONE/SHORTWAVE OVER CENTRAL ND MAY CONTINUE TO BE THE
GREATEST AREA FOR HIGHEST TOTALS GIVEN THE NEAR STATIONARY CELL
MOTION...THOUGH MOISTURE FLUX WILL BE THE TRADE-OFF  DUE TO
AFOREMENTIONED UPSTREAM CONVECTIVE COMPLEX IN NE SD ROBBING BEST
INFLOW.

GALLINA

ATTN...WFO...ABR...BIS...FGF...FSD...

ATTN...RFC...MBRFC...NCRFC...

LAT...LON   48020030 47189859 46619778 45359687 44109770
            43709933 44529988 45479971 46559991 47490077