Anticipating Heavy Rainfall:Forecast Aspects

By

Richard H. Grumm* National Weather ServiceState College PA 16803

andRobert Hart

The Pennsylvania State UniversityUniversity 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

Methoddata

• Initial data : NCEP CD-ROM

• Current Data (1 Jan 1948- 31 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

Methodstandardized 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 Caselook for outliers

• Examined Uccellini-Kocin snowstorms– not many were historic events– most varied 2-3 Standard deviations from normal

• computed top 20 anomalous events 1948-2000• 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: http://eyewall.met.psu.edu/

Heavy Rain eventsMAFC MAP big eventsOriginal Tables:

Mid-Atlantic River Forecast Center Largest 24-h rainfall events1994-2000

Date Q850 V850 H500 V250 Notes

27-29 Jul 94 +1 +3 -3 +3 Deep SSW flow13-14 Aug94 +1 +3 _ +2 E-W frontal boundary to north

Strong 850 hPa easterly jet17-19 Aug94 +2 +3 _ +2 Tropical Storm Beryl connection

Southeasterly low level jet27-28 Nov94 +3 +3 -2 +2 Strong low level easterly jet22-23 Jun95 +1 +1 +1 +2 Low level easterly jet -2 to -3 departures

Deep southeasterly flow27-28 Jun95 +1 +3 _ +1 Low level easterly jet -3 departures

Deep southeasterly flow10-11 Jul95 _ _ -1 _ Weak flow case20-21 Oct95 +2 +4 -3 +2 Narrow cold frontal rainband case17-18 Jun96 - +1 _ * Strong N-S temperature gradient-Maddox MCS/MCC

*Strong upper level easterly jet18-19 Jun96 +1 +2 _ _ Strong N-S temperature gradient-Maddox MCS/MCC

Strong easterly LLJ just north of heavy rain area8-9 Aug96 +1 +1 _ _ Southerly flow with upper level jet entrance over

region5-7 Sep96 +2 +5 +1 +2 Strong N-S temperature gradient

Strong (-4) LLJJet Entrance circulation caseTropical Storm Fran

18-19 Oct96 +2 +2 -3 +2 Deep southeasterly flow -unidirectional shear case08-09 Nov96 +3 +4 -2 +3 Narrow cold frontal rainband case01-02 Jun97 +1 +1 -3 +5 Strong (-5) LLJ

Heavy rain NNE of H500 cut-off low01-02 Jul97 +1 +1 -1 -2 Weak case10-11 Sep97 +1 +3 -3 +3 Early season narrow cold frontal rainband case

Deep southerly flow-unidirectional shear case07-08 Nov97 +1 +1 -3 +1 Heavy rain NNE of H500 cut-off low

Strong (-4) LLJModerate (-2) ULJDeep southeasterly flow

07-08 Jan99 +5 +4 -3** +2 **H500 low well to southwest+2 H500 Ridge over northern New EnglandStrong N-S temperature gradient over New EnglandRecord ice event in northern New England andCanada

6-7 Sep99 +2 +2 _ +1 Deep southeasterly flow undirectional shear case15-17 Sep99 +3 >5 -3 +3 Tropical connection with rements of Floyd

Strong (-4) easterly flow in upper level jet17-18 Apr00 +1 +1 _ +1 Moderate (-2) easterly LLJ case

Heavy Rain eventsNarrow cold frontal rainbands

Narrow Cold Frontal Rainband Cases

Date Q850 T850 V850 H500 Notes

25 Nov 1950 +2 * +4 -4 Record November eventPreceeded by strong event of 20 Nov 1950Bristor (1951)8th most anomalous November case since 1948*negative thermal anomaly behind the cold front inexcess of -5

21 Nov 1952 +4 +1 +3 <-5 Identified from historic stream flow dataStrong event largest anomalie at 500hPa3rd most anomalous November case since 1948

19-20 Nov 1954 +3 +2 +1 -4 10th most anomalous November case since 194801 Apr 1987 +4 +3 +4 -4 2nd most anamalous April event since 1948

Budd(1988)21 Oct 1995 +2 +1 +4 -311 Nov 1995 +3 +1 +4 -319 Jan 1996 >5 +2 +4 -2 6th most anomalous January event since 1948

Historic snowmelt and floodingLeathers et al. (1998)

24 Jan 1996 +2 +1 +3 -227 Jan 1996 +2 +1 >5 -2 3rd event of the same type in 8 days18 Oct 1996 +2 +1 +2 -308 Nov 1996 +3 +1 +4 -3

Table 2 As in Table 1 except data for narrow cold frontal rainband events. Data include the 24-h period,the 850 hPa specific humidity (Q), warm sector temperatures (T), and southerly wind (V) anomalies; andthe 500 hPa height amomaly (H).

Heavy Rain events19 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 (oC), c) 850 hPa V-winds (kts) and d) 850 hPa dew points (oC) and specific humidity standardized anomalies. Height contours are every 6 dm, isotherms every 2 oC, winds everyt 5kts, and dew points every 4 oC. Standardized anomalies are shown by the key to the right of each panel.

Heavy Rain eventscut-off low types

Cut-off low TypesDate Q850 U850 V850 H500

02 Jun 1997 +1 -4 +1 -3 to-4

08 Nov 1997 +1 -4 -- -315 Jul 2000 -- -4 +4 -412 Aug 2000 -- -4 -- -412 August event spanned 12-14 August. Heaviestlocal rainfall was on the 12th.

Heavy Rain eventsAugust 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 Applicationsensembles

• 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 Applicationsensembles

Forecast Applicationsanticipating record heat wave-Nov 1999

Short term modelsheavy 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 1999. 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 modelsheavy 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 modelsheavy rainfall examples-12 August 2000

• Thickness anomalies– 1000-500 hPa cold– 1000-850 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 2000. Data shown include 1000-500 (upper) and 1000-850 hPa thickness values and the standardized anomalies.

Short term modelsheavy 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 modelsheavy rainfall examples-12 August 2000

• 700 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 4oC.

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...