Anticipating Heavy Rainfall: Forecast Aspects By Richard H. Grumm* National Weather Service State College PA and Robert Hart The Pennsylvania State University University Park,PA 16804

Introduction Data and visualization methods to facilitate identifying heavy rainfall events Climatological aspects –historical events –event typing –statistical analsis Forecast aspects –apply parameters to model forecasts –apply parameters to ensemble consensus

Method data Initial data : NCEP CD-ROM Current Data (1 Jan August 2000) –NCEP re-analysis data –key fields mandatory level data –heights, temps, u and v winds –mslp –derived : thickness values from height fields ability to display and examine key fields real-time access to data

Method standardized anomalies Intial Goal –examine monthly standardized anomalies –changed to 30-day centered anomalies –files of means and standard deviations for each parameter GRADS –compute means and standard deviations –REGRID anomaly data to model fields and ensembles

Historical Case look for outliers Examined Uccellini-Kocin snowstorms –not many were historic events –most varied 2-3 Standard deviations from normal computed top 20 anomalous events found top 10 by month began literature search and on-line displays of cases –Junker 1993 flood cases began typed climatologies –Paper accepted MWR August 2000 –Website:

Heavy Rain events MAFC MAP big events

Heavy Rain events Narrow cold frontal rainbands

Heavy Rain events 19 Jan historic flooding Figure 1. Reanalysis data valid 00Z 19 January 1996 showing observed fields and the standardized anomalies of a) 500 hPa heights, b) 850 hPa temperatures ( o C), c) 850 hPa V-winds (kts) and d) 850 hPa dew points ( o C) and specific humidity standardized anomalies. Height contours are every 6 dm, isotherms every 2 o C, winds everyt 5kts, and dew points every 4 o C. Standardized anomalies are shown by the key to the right of each panel.

Heavy Rain events cut-off low types

Heavy Rain events August 2000 Figure 2. As in Fig.1 except valid at 0000 UTC 13 August 2000 showing a) 500 hPa heights, b) 850 hPa isotherms, c) 850 hPa U-winds, d) 850 hPa V-winds.

Forecast Applications ensembles Show ensembles in traditional spaghetti plots show ensemble consensus and standardized anomalies –view potential significant weather events –focus on key fields and key parameters Early success Oct 99 cold outbreak and Nov 99 record plains heat

Forecast Applications ensembles

Forecast Applications anticipating record heat wave-Nov 1999

Short term models heavy rainfall examples-20 August 1999 U-wind components –anomalous east flow –moisture off Atlantic V-winds –strong southerly winds Localized terrain focused heavy rainfall Figure 1. Twelve hour forecasts from the Eta model valid 1200 UTC 20 August Data shown include the forecast wind speed and departures of U-component and V- components in the upper and lower panels respectively. Departures represent the number of standard deviations these wind components depart from the 50-year climatological values.

Short term models heavy rainfall examples-15 July 2000 U-wind components –anomalous east flow –moisture off Atlantic V-winds –strong southerly winds Banded heavy rain event Figure 2. As in Fig. 1 except for the Eta 06 hour forecast valid 1800 UTC 15 July 2000.

Short term models heavy rainfall examples-12 August 2000 Thickness anomalies – hPa cold – hPa not too cold –unstable situation –coldest anomaly in mid-levels Figure 3. As in Figure 1 except Eta 12-hour forecasts valid 0000 UTC 13 August Data shown include (upper) and hPa thickness values and the standardized anomalies.

Short term models heavy rainfall examples-12 August 2000 Winds –no real southerly component –strong easterly component Figure 4. As in Fig. 1 except Eta 9-hour forecast valid 21 August 2000.

Short term models heavy rainfall examples-12 August and 850 hpa Q –not anomalous –long fetch over Atlantic toward terrain Heavy record rainfalls NJ/srn NY Figure 5. As in Fig. 3 except for Eta 12-hour forecasts of 700 and 850 hPa dew points and the specific humidity departures in standard deviations from normal. Dew points contours are every 4 o C.

Conclusions Clear signal in record events in the 52 year climatology Easy to recreate any case Standardized anomalies have great forecast potential –see significant events in ensembles –see significant events in Eta (shown) and AVN –need to learn key features with key parameters Work in progress…but we learn as we share...