Notes: Heavy Rainfall Forecasting
Heavy Rainfall Forecasting Techniques
Theodore W. Funk
Central Region Tech. Attachment (1993)
-No one method can be utilized by itself without consideration of all other parameters.
-Parameters/Techniques - Examples of pattern recognition; Synoptic, Mesohigh, cyclonic circulation, and SHARS - Subtle heavy Rainfall Signatures.
-Moisture availability - high ambient and/or inflow moisture must be present and maintained. Precipitable water (PW) values greater than one inch and at or above normal. K index values (ambient or inflow) 30 to 40 or more. 850 mb and surface dew points (ambient or inflow) near or especially above 12oC and 17oC (60 oF) respectively (warm season)
-Low-level Inflow and Convergence - Moderate to strong moist surface to 850 mb inflow (10 kts or more at the surface, 25 kts or more at 850 mb). Such persistent southerly inflow converging toward a quasi-stationary low-level frontal or outflow boundary can signify the potential for rainfall amounts approaching or exceeding 5 inches in a 24 hour period.
-Jet stream structure - favored locations:1)right entrance, 2) left exit region, 3) exit region of a jet streak approaching the top of a ridge axis, 4) area of upper level divergence, 5) anticyclonic shear axis to the right of a jet core. coupled upper-level jet streaks (convection within the right entrance of the polar jet and left exit of the subtropical jet simultaneously). Upper-level/lower-level jet coupling through direct and indirect circulations.
-Warm air advection - Associated heavy precipitation occurs most often at night and in the early morning. Associated with the exit region of the low-level wind maximum. Heavy precipitation potential exists if model forecasted thickness or 850 mb temperatures hold steady or sink southward in the face of southerly warm air advection, since the warm air is being lifted instead of actually warming the air at a particular level.
-Theta-E - low level (especially 850mb) ridge axis coincident with upward motion and unstable air is a prime location for convective development. In warm air advection/overrunning situations, a region of positive advection of theta-e by the low-level wind is where overrunning convection will be found. Tight gradients of theta-e (baroclinic situations) are favored for heavy precipitation.
-Thickness Diffluence - Implies low-level convergence or upper-level divergence, but most likely a combination of the two, and, therefore, is an area conducive for convective development.
-Thickness saturation - When the ambient or inflow PW's represent at least 70 percent saturation of the ambient 1000-500 mb thickness. Sufficient moisture must be present to cause saturation in a diffluent thickness region, or else convection likely will form farther north (downwind) within tighter thickness packing, as in overrunning situations.
-Preferred Thickness- Useful tool for determining the location of initial convection when forcing mechanisms are weak.
-Rules of Thumb - Large volume convective rainfall tends to occur farther south or southeast with time over the central U.S. if outflow boundaries from current or previous convection can intercept moist southerly flow.
Heavy rainfall producing convection often develops within or along the upstream edge of a vorticity minimum ridge axis at 500 mb. Watch for convection behind a weak shortwave if moist unstable inflow persists into a low-level boundary. The convection then is maintained by low-level forcing. If a well-defined middle and high level tropical moisture connection exists in water vapor imagery, rain potential is typically higher than normal. Inverted isobars signal the possibility of heavy rainfall. Models are subject to "convective feedback" shortwaves, which models induce through deep convection, strong vertical velocities, and latent heat release. Models then generate subsequent precipitation (often bullseyes).
Elevated Thunderstorms Associated with Heavy Rainfall in The Midwest
J.T. Moore, S.M. Rochette, F.H. Glass, D.L. Ferry, & P.S. Market
Preprints 18th Conf. of Severe Local Storms, 1996 San Francisco, CA Amer. Meteor. Soc.
-Introduction - Colman (1990a,b) notes that elevated thunderstorms are isolated from surface diabatic effects and occur above frontal surfaces. Elevated thunderstorms with attendant heavy rainfall often fit the synoptic patterns described by Maddox et al. (1979) as the "frontal" or "mesohigh" type flash flood scenarios.
-Methodology - Augustine and Caracena (1994) showed that large, long-lived MCSs tend to form overnight in the region where the LLJ encounters a frontogenetic region at 850 mb, north of the surface boundary. Small MCSs were associated with a weaker front, if present, which was not subject to frontogenetical forcing. Frontogenetical forcing typically enhances the direct thermal circulation pattern and thus the subsequent convective activity. Glass et al. (1985) found the favorable region of heavy convective rainfall as being north of a west-east surface boundary, and south of the positive 850 mb theta-e advection maximum that is coupled with a southerly LLJ. This region is especially favored if the surface boundary is quasi-stationary and the 850-300 mb thickness field is diffluent. In this study, each of the seven cases formed in a region of elevated convective instability (i.e., a region above the stable boundary layer where theta-e decreases with height).
-Results - Moisture transport vectors, computed as the product of the vector wind and the mixing ratio, clearly demonstrate that heavy rainfall producing MCSs are favored about 400 km downstream and to the east of the maximum horizontal transport of moisture. The 500 mb composite height and vorticity analysis reveal a weak short wave trough upstream from the MCS initiation point with weak PVA.
-Conclusions - MCSs with elevated thunderstorms associated with heavy rainfall intiated...
1) About 200 km downstream from the LLJ maxima
2) In a region characterized by maximum strong theta-e advection and moisture convergence at 850 mb
3) About 400 km downstream and to the east of the maximum moisture transport vectors at 850 mb
4) In a region of anticyclonic curvature and downstream of a weak 500 mb short wave trough
5) In the right entrance region of the 200 mb ULJ, near the divergence maxima.
6) In a region characterized by a stable LI, a slightly unstable SI and having a relatively high value of the KI and elevated convective instability between 500 and 850 mb
7) In a region of >60% mean surface to 500 mb relative humidity and a precipitable water >1.2 in.