A Synoptic And Mesoscale Analysis Of Heavy Rainfall At .

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Curtis, M. B., 2017: A synoptic and mesoscale analysis of heavy rainfall at Portland, ME 13-14 August 2014. J. OperationalMeteor., 5 (7), 78-86, doi: https://doi.org/10.15191/nwajom.2017.0507.A Synoptic and Mesoscale Analysis of Heavy Rainfallat Portland, Maine, 13-14 August 2014MARGARET B. CURTISNOAA/National Weather Service, Gray, ME(Manuscript received 30 June 2016; review completed 15 December 2016)ABSTRACTPortland, Maine, experienced record setting rainfall of 163.3 mm (6.43 in) on 13–14 August 2014, whichresulted in urban flash flooding. This paper examines the key ingredients for heavy convective rainfall: highspecific humidity, intense vertical motion, and long rainfall duration at both the synoptic scale and mesoscale. Afavorable environment for flooding is identified from positive anomalies in the specific humidity and a synopticscale, upper-level trough just upstream of Portland. The existence of a low-level jet altered the propagation ofthe responsible convective system as it approached Portland. This jet resulted in a change in the mesoscale betaconvective element propagation vector (VMBE). The concept of residence time as a function of the angle betweenthe synoptic motion and VMBE is introduced and found to increase the duration of heavy rainfall in this case. Inaddition to the effects attributable to system motion, a coastal front (more commonly a winter phenomenon)is also identified in this case. This front enhanced the vertical motion, further increasing the local rainfallmaximum over the Portland area.1. Introduction(11.74 in) (Menne et al. 2012). Even though the 13-14August 2014 total rainfall was not record setting, thehigh rainfall rates resulted in widespread urban flooding.Figure 2 shows the rainfall recorded by the AutomatedSurface Observing Station (ASOS) at the PortlandInternational Jetport (KPWM). The 1-min rainfall, 15min average rainfall rate, and total accumulation areplotted. Most of the rain fell between 0100 and 0300UTC 14 August 2014 (9–11 pm EDT), and rainfall ratespeaked at more than 110 mm hr–1 (4.33 in hr–1).According to Doswell et al. (1996), the keyingredients for heavy precipitation are high rainfallrates and long rainfall duration. Variables importantfor producing high rainfall rates include precipitationefficiency (E), high specific humidity (q), and strongupward vertical motion (w) (Doswell et al. 1996).Rainfall duration is dependent on the motion of boththe individual cells and the system as a whole. Rainfallis maximized when multiple cells within a cluster passover the same location (Doswell et al. 1996). Jessupand Colucci (2012) examined mesoscale convectivesystems resulting in heavy rainfall in the NortheastUnited States and found back-building features areHeavy rainfall and flash flooding impacted thePortland, Maine, metro region on the evening of 13August 2014. The total amount recorded was 163.3 mm(6.43 in). This set a new daily maximum rainfall recordfor 13 August and also ranked as the 5th greatest 24-hrrainfall at Portland. Furthermore, most of the rainfall,more than 101.6 mm (4 in), fell in just two hours.This case was selected because of the historic natureof the rainfall. In addition to the flooding in Portland,Islip, New York, also set a 24-hr rainfall record forNew York State several hours before rainfall beganin Portland. In southern Maine, heavy precipitationoccurred throughout the evening of 13 August 2014.Figure 1 shows the rainfall received across the region.Even though a large portion of southern Maine andNew Hampshire received more than 50.8 mm (2 in) ofrain, the highest amounts were focused in the Portlandmetro region where 152–203 mm (6–8 in) fell. Thesevery large rainfall amounts resulted in urban flashflooding and caused 1.5 million in damage for theregion (NCDC 2014).The 24-hr rainfall record for Portland is 298.2 mmCorresponding author address: Margaret B Curtis, NOAA/NWS, P.O. Box 1208, Gray, ME 04039E-mail: Margaret.Curtis@noaa.gov78

Curtis, M.B.NWA Journal of essure created a broad region ofupward motion over southern Maine. There was a broadarea of upward vertical motion analyzed in the NARR,with a peak value of 1.5 Pa s–1 over the Portland area,satisfying one of the favorable ingredients for heavyrainfall identified by Doswell et al. (1996).In addition to a relatively strong low pressuresystem, high specific humidity (q), was also present.Figure 3 shows the specific humidity at several levelsat 0000 UTC 14 August 2014, the time of the heaviestprecipitation at Portland. The specific humidity was oneto two standard deviations above the average throughoutall vertical levels.The 0000 UTC 14 August 2014 sounding from Gray,Maine (KGYX, Fig. 4) highlights several additionalaspects conducive to heavy rainfall. The lowest 5 kmof the sounding was above freezing and completelysaturated, providing a perfect environment for warmrain (collision-coalescence) dominated processes. Thishelped improve the precipitation efficiency contributingto the heavy rainfall (Davis 2001). The sounding had47.41mm (1.86 in) of precipitable water, which is abovethe 90th percentile of KGYX soundings for the date.Both the significant deep warm cloud layer and theamount of moisture contributed to the heavy rainfall.Maddox et al. (1979) identified a synoptic type floodevent characterized by a strong low-level jet parallel toa surface front, and an associated strong short-wavetrough at 500 hPa. Figure 5 depicts the NARR analysisduring this flood event. A 500 hPa short-wave troughwas digging into the Northeast United States. The areaISSN 2325-6184, Vol. 5, No. 715 June 2017b. Storm motionThe prior section demonstrated the synopticsituation was ripe for a flood event. An anomalouslow pressure system was aiding in transporting verymoist air northward into the region, yielding significantmoisture and upward motion necessary for heavyrainfall. The second component of heavy rainfall isevent duration. The duration of events can be enhancedby back building. Jessup and Colucci (2012) identifiedseveral types of back building events associated withheavy rainfall in the Northeast United States. This casemay be best characterized as being one in which a linearfeature was followed by back-building convection.A linear convective system or squall line waspresent over western Maine at 0100 UTC 14 August2014 (Fig. 6). While the line’s motion was from west toeast, individual cells within the line moved from southto north, with average 0–6 km storm motion of 180º at14.9 m s–1 (29 kt) (Calculated using the 30R75 methodof Maddox 1976, based on the 0000 KGYX 14 August2014 sounding). To determine the rate of propagation,the mesobeta-scale convective elements (MBE) vectors(VMBE) for this system were calculated according toCorfidi et al. (1996).As the system leaves New Hampshire there is littleto no low-level jet present with the 850 hPa flow on theRAP13 analysis from 160º at only 10.2 m s–1 (20 kt) (Fig.7). With a system motion from west to east and the 0–6km storm motion vector of 180º at 14.9 m s–1 (29 kt), wesee the orientation of the system’s cold pool and gustfront are perpendicular to the system motion resultingin a downwind propagation scenario. Following Corfidi(2003) and combining the upwind propagation vectorwith the mean 300–800 hPa wind, this results in a VMBEof 180º at 23.2 m s–1 (45 kt). MBEs within the line areracing quickly to the north, but despite extremely highrainfall rates, no significant flooding is reported.When the squall line approached Portland, itencountered a very strong low-level jet and a coastalfront that both impact system motion. Consider thedynamics of the system, which prior to arrival inPortland had been a strongly forward propagating80

Curtis, M.B.NWA Journal of Operational Meteorology(a)(b)(c)(d)15 June 2017Figure 3. Specific humidity, q, (black lines, g kg–1) and anomaly (shaded, g kg–1) at 0000 UTC 14 August 2014, for(a) 500 hPa (b) 700 hPa (c) 850 hPa (d) 925 hPa from the NARR.MCS. The motion of the convective line is governedby advection and propagation components (Corfidi2003). The propagation component is influenced by themotion of the cold pool and the strength of the lowlevel jet, both of which change as the system moves intoPortland.The motion of a squall line is dependent, in part,on the system’s cold pool and associated gust front. InISSN 2325-6184, Vol. 5, No. 7this case it appears a coastal front (or, more properly,the cool-air dome to its north) augmented the systemcold pool to create an enlarged, and much elongated,effective cold pool than previously was present. Moreimportantly than the size of the cold pool/coastal front,the orientation of the coastal front (approx. 240º to60º) meant the system’s effective cold pool was nolonger perpendicular to the mean wind but now had81

Curtis, M.B.NWA Journal of Operational MeteorologyFigure 6. KGYX reflectivity 0.5 tilt and RAP13analysis of 300-800 hPa mean wind (red) and 850 hPaflow (black, kt), upwind MBE motion (pink, kt) anddownwind MBE motion (yellow, kt) for 0100 UTC 14August 2014, when the system crossed Portland, Maine.Locations of the close-up in Fig. 7 indicated by stars.Figure 4. Sounding from Gray, ME for 0000 UTC 14August 2014 (wind barbs, kt).(a)(b)Figure 7. RAP13 analysis of 300-800 hPa mean wind(red) and 850 hPa flow (black), upwind MBE motion(pink) and downwind MBE motion (yellow) for 0100UTC 14 August 2014 for New Hampshire (a), andPortland, Maine, (b). Locations are indicated in Fig. 6.Figure 5. 0000 UTC 14 August 2014 North AmericanRegional Reanalysis data showing 500 hPa height(bold solid), 850 hPa wind (kt, wind barbs), surfacedewpoint ( C, dashed lines). Area favorable for flashflood formation based on Maddox et al. (1979) includesdownstream from 500 hPa trough axis (blue dashedline) in proximity to 850 hPa wind maxima (red arrow).storm motion. Prior to line’s approach, storm motionderived from the 00Z KGYX sounding was 180º at 14.9m s–1 (29 kt); however, the 850 hPa jet increased to 25 ms–1 (49 kt) from 135º, at Portland, giving an VMBE of 280ºat 18.5 m s–1 (36 kt). The change in the direction andincrease in speed of the low-level jet resulted in systempropagation evolving to one exhibiting an increasingdegree of backward or upstream development, ratherthan the mainly forward propagation seen prior to theentrance into the low-level jet.Because sounding data was not available across theregion, RAP13 analysis is used to examine the spatialvariability in the VMBE. The spatial changes in the low-a component parallel to the mean wind, resulting ina back-building system. Further, the coastal frontprovided a more extensive source of enhanced lowlevel convergence than the system’s cold pool alone.Having established the system was back-buildingas it entered the Portland area, the methods of Corfidi etal. (1996) for backward-propagating systems can nowbe utilized to determine the propagation calculated bysubtracting the 850 hPa low-level jet from the meanISSN 2325-6184, Vol. 5, No. 715 June 201782

Curtis, M.B.NWA Journal of Operational Meteorologylevel jet play a significant role in enhancing the backbuilding by changing the magnitude and direction ofVMBE. Consider the spatial variability of the low-leveljet and VMBE from RAP13 shown in Fig. 6. While theforward propagation vector (yellow) remains largelyout of the south for the entire region, the backwardspropagation vector (pink) turns from being southerlyand perpendicular to the line motion in New Hampshireto partially parallel to the line motion in Portland (Fig.7). Throughout the region, the magnitude of VMBEremains similar at 7.7–10 m s–1 (15–20 kt) for backwardpropagation, and around 25 m s–1 (50 kt) for forwardpropagation. Normally, a large-magnitude VMBE isassociated with fast-moving systems and no flooding.However, in this case, the change from forward tobackward propagation and the accompanying change inthe direction of VMBE — and not just VMBE magnitude— are important in determining the predominant threat(excessive rainfall) posed by the MCS.Examining the motion of the convective line asa whole, as determined from timing the leading edgeof the precipitation echoes from the KGYX radar, theline moved from 270º at 10.8 m s–1 (21 kt). Takingthis motion into account we can see the effect of thechanging VMBE. As the line enters the region where thelow-level jet is the strongest, the direction of the MBEmotion changes from perpendicular to the line motion topartially parallel to the line motion. This has the effectof increasing the length of time individual cells remainover the region, providing for the second component ofheavy rain — a relatively long duration compared towhen the system was moving through New Hampshire.The impacts of spatial variability of MBE motionseen in the RAP13 analysis of this event in Fig. 6,can be idealized. Consider the schematic in Fig. 8. Inscenario (a), what was seen initially in New Hampshire,timing the leading edge of the precipitation echo showsthe convective line is moving from the west and theMBEs are moving from the south twice as fast. As timeprogresses, only one MBE, (denoted ‘A’ in the figure)is able to pass over the point of interest, X. In scenario(b) the line encounters the low-level jet near Portland.The line movement remains the same, but owing to thepresence of the enhanced low-level jet, VMBE becomeswest-southwest. The means the MBE motion is nowpartially parallel to the line motion, which sets up ascenario for training. In scenario (b) MBEs A, B and Care able to pass over point X. Typically long residencetimes of individual MBEs over a region are associatedwith very slow VMBE. However, this case demonstratesthe importance of the orientation of VMBE relative to theISSN 2325-6184, Vol. 5, No. 715 June 2017Figure 8. Effect of changing the orientation of theMesoscale Beta element vector (VMBE) and convectiveline motion (Vl) for a series of convective MBEs (A,B, C) passing over location (X). In (a) VMBE and Vl areperpendicular, resulting in only cell A passing overlocation X. In (b) VMBE has a component parallel to Vlresulting in the line changing its orientation and cells A,B, and C all passing over location X.system motion. While the magnitude of VMBE remainsfairly large, the orientation of the VMBE to the convectiveline motion results in a long residence time.The concept of residence time for any convectivesystem can be formulated more generally. For anysystem with velocity Vl and an environment with VMBE,the increase in time spent over a region because of adecrease in the angle between the line motion and theMBE motion, can be expressed via the component ofthe VMBE parallel to the line motion (projV1VMBE) ascompared to the line motion, proposed as the ‘residencefactor’ (tR) and expressed as:(1)For this case, the line motion is 270º at 21 kt, givingtR of 1.6 as the increase in time because of the alteredorientation. Thus, Portland experienced high rainfallrates for over 2 times longer (1 tR) than any precedinglocation the line passed because of the changes in MBEdirection upon entering the low-level jet.c. The coastal frontThus far, it has been demonstrated that highspecific humidity, high precipitation efficiency, andthe potential for long duration and back buildingconvection contributed to the formation of heavy rain.83

Curtis, M.B.NWA Journal of Operational MeteorologyTable 1. Blocking parameters computed for the KGYX0000 UTC 14 August 2014 sounding. U is positivetowards 300 degrees, and V is positive towards 30degrees.The final major ingredient required to produce heavyrain is upward motion. In this case, upward motion wasmaximized by the presence of a coastal front.New England coastal fronts may be formed inseveral ways. Once such area favorable to coastalfront formation is “ beneath the forward side ofadvancing troughs near mountain barriers whereupslope flow results in differential airmass cooling andstabilization ” (American Meteorological Society2015). In this case Fig. 5 shows the 500 hPa height field,with the area around Portland in the favored region forcoastal front formation ahead of an advancing upperlevel trough.Following the calculation method of Nielsen(1989) the potential for frontogenetical forcing tobe generated by the presence of warm advection in arotating stratified flow past orography is considered(Gardner 1986). Table 1 lists the relevant parametersfor the lowest layer observed in the Gray, ME soundingfrom 0000 14 August 2014. Overall the lowest warmadvection layer was found to have Froude number (Fr)1.2 and Rossby number (Ro) 3.33, falling within the 1 Fr 1.5 and Ro 2 range identified as characteristicof orographic blocking enhancing frontogenesis inNew England. Thus, the environment favored not onlysynoptic signals for heavy rainfall, but also coastal frontformation.A coastal front formed ahead of the low pressurecenter. Figure 9 shows the location at 2200 UTC13 August 2014. The front extends from easternMassachusetts northeastward across coastal Maine. Theinland location of this front along with the orographicblocking parameters characterize it as a type-C coastalfront (Nielsen 1989); in these cases coastal frontogenesisis found to take place away from the coast with minimalland-sea differences. At 2200 UTC 14 August 2014 theair temperatures along the coastal plain were around17 C (63 F), while the water temperature at 44007 buoy(19 km southeast of Portland Jetport approximately 5km offshore) was 16.6 C (62 F), yielding almost noland sea contrast.The main role of the coastal front is to providethe upward vertical motion necessary to sustain thehigh rainfall rates. Figure 10 shows a radial velocitycross section of the coastal front near Portland takenfrom the KGYX radar just before the convective lineof precipitation moved into the area. From this crosssection we can identify the frontal slope as the zoneof convergent winds indicated from the radar. Theconvergence is shown in Fig. 10 as the junction of theoutbound velocity (red) and inbound velocity (green)ISSN 2325-6184, Vol. 5, No. 715 June 2017Figure 9. 2200 UTC 13 August 2014 surface windobservations (kt, wind barb) from mesonet, airport, andmarine locations, and isotherms (blue, dashed) fromRAP analysis. The approximate location of the coastalfront across New England is indicated.beginning at the surface just northwest of Portland andextending upward and inland toward North Windham.This frontal surface extends 1.5 km aloft over a distanceof approximately 16 km, giving a slope of 5.3º. The flowup and along the frontal surface is 25 m s–1, yielding84

Curtis, M.B.NWA Journal of Operational Meteorology15 June 2017an estimated upward velocity component of 2.4 m s–1.d. Heavy rainfallRainfall rates are directly proportional to theupward velocity and the specific humidity (Doswell etal. 1996). In this case, upward velocity was created byboth synoptic scale rising motion and the coastal front.In the vicinity of the coastal front, the 2.4 m s–1 upwardmotion associated with the boundary, along with 1.5 ms–1 ascent provided by the synoptic-scale pattern andthe 12 g kg–1 mixing ratio air at 925 hPa, can be usedto determine the potential precipitation rate (P). Theprecipitation rate is proportional to the Precipitationefficiency (E), specific humidity (q), and upward motion(w). (Doswell et al. 1996)P Eqw(2)Figure 10. 50 km (32 mi) velocity data (kt, colored)cross section perpendicular to the coastal front atPortland, Maine, 0000 UTC 14 August 2014 fromKGYX velocity data. Vertical scale is 3.05 km (10 000ft). The location of the cross-section is noted in Fig. 11.Assuming an idealized precipitation efficiency of 1yielded potential precipitation rates in excess of 200 mmhr–1. Without the coastal front’s contribution of 2.4 ms–1 upward motion, the resulting rainfall rates would beexpected to be third as much. Examini

Curtis, M.B. NWA Journal of Operational Meteorology 15 June 2017 and Rapid Refresh (RAP13) analysis (Benjamin et al 2016) were used to examine synoptic and mesoscale features. 3. Analysis a. Synoptic overview One of the contributors to the heavy rainfall was a synoptic-scale low press

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