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...