g | x | w | all
Bytes Lang Time Link
255Nim250312T061812ZArchar G
220Python 2150130T095938ZLogic Kn
150PHP 150 Characters141006T043606ZTyzoid
218Mathematica121002T235320ZDavidC
251Javascript121002T160555Zcodeporn
095Ruby 1.8110210T215217ZVentero
104Windows PowerShell110203T231602ZJoey
110Golfscript110211T205006ZYOU
288PHP110205T215204ZKevin Br
350C# 350 Characters110203T160357ZNellius
436D 436 Characters110203T121557ZJonathan

Nim, 261 255 bytes

include prelude,random;randomize()
var
 d="Mon Tues Wednes Thurs Fri Satur Sun".split
 p=stdin.readline.split.mapit it[0..1]
for w in 0..6:echo d[(w+mapIt(d,it[0..1]==p[0]).find on)%%7],"day ",20.rand+{"Sp":10,"Su":20,"Au":5,"Fa":5,"Wi": -5}.toTable[p[1]]

Try it online on glot.io

Ungolfed:

include prelude, random
randomize()
var
  days = "Mon Tues Wednes Thurs Fri Satur Sun".split
  input = stdin.readline.split.mapit it[0..1]
for weekday in 0..6:
  let dayOffset = mapIt(days, it[0..1] == input[0]).find on # `on` is alias for `true`
  let tempOffset = {"Sp":10,"Su":20,"Au":5,"Fa":5,"Wi": -5}.toTable[input[1]]
  echo days[(weekday + dayOffset) %% 7], "day ", 20.rand + tempOffset

Python 2, 220 characters

A little large but (almost) readable.

import random
c,e=raw_input().split()
a=[d+'day'for d in'Mon Tues Wednes Thurs Fri Satur Sun'.split()]*2
x='rmtn'.index(e[2])*2
g=(10,30,20,40,5,25,-5,15)[x:x+2]
f=a.index(c)
for b in a[f:f+7]:
 print b,random.randint(*g)

Output

# echo "Monday Summer" | python forecast.py 
Monday 33
Tuesday 29
Wednesday 33
Thursday 28
Friday 25
Saturday 21
Sunday 30

PHP - 150 Characters

<?$t=array("m"=>15,"r"=>5,"n"=>-10);for($a=split(" ",fgets(STDIN));$i<7;$i++)echo date("l ",strtotime($a[0]."+$i day")),rand(5,25)+$t[$a[1][2]],"\n";

I figured I'd write my own PHP solution after the current one doesn't even fully satisfy the challenge conditions.

It relies on strtotime to parse the day, and date to echo it back. To determine the season, it follows the third letter of the season name, which is unique (as given).

To run properly, it requires notices to be disabled and short tags to be enabled.

Mathematica 218

s_~g~d_ := 
Grid@RotateLeft[r = Thread@{DateString[{2012, 1, #}, {"DayName"}] & /@ Range@7, 
  RandomInteger[Cases[{{"Spring", {10, 30}}, {"Summer", {20, 40}}, {"Autumn", {5, 25}}, 
  {"Winter", {-5, 15}}}, {s, _}][[1, 2]], 7]}, Position[r, d][[1, 1]] - 1]

Usage

g["Winter", "Sunday"]

enter image description here

Javascript - 251 chars

d=['Mon','Tues','Wednes','Thurs','Fri','Satur','Sun'],g=function(c){return Math.random()*21+c|0},p=prompt();for(i=0,j=7,t=true;i<j;i++){t=t&&p.indexOf(d[i])!=0;t?j++:console.log(d[i>6?i-7:i]+'day '+g(/mm/.test(p)?20:/g/.test(p)?10:/te/.test(p)?-5:5))}

Unfortunately the script does not meet the stdin/stdout requirement but it accepts Fall as a synonym for Autumn.

Whitespaced:

  d = [
    'Mon'
  , 'Tues'
  , 'Wednes'
  , 'Thurs'
  , 'Fri'
  , 'Satur'
  , 'Sun'
  ]
, g = function(c) {
    return Math.random()*21+c|0
  }
, p = prompt()

  ;

  for(i = 0, j = 7, t = true; i < j; i++) {
    t = t && p.indexOf(d[i]) != 0; 
    t ? 
      j++ : 
      console.log(d[i > 6 ? i - 7 : i] + 'day ' + g(/mm/.test(p) ? 
                                                      20 : 
                                                    /g/.test(p) ? 
                                                      10 : 
                                                    /te/.test(p) ? 
                                                      -5 : 
                                                      5
                                                    )
                  )
  }

Ruby 1.8, 95 characters

#!ruby -nrdate
7.times{|i|puts (Date.parse($_)+i).strftime"%A #{"\023\004\016\016\035"[$_[-3]%5]-9+rand(21)}"}

The character escapes inside the string should be replaced by the character literals they represent.

Windows PowerShell, 104

[dayofweek]$d,$s=-split$input
(0..6*2)[+$d..(6+$d)]|%{''+[dayofweek]$_,((random 20)-5+'☼

↓'[$s[3]%5])}

The strings in there are a bit icky, so a hex view for your convenience:

000: 5B 64 61 79 6F 66 77 65 │ 65 6B 5D 24 64 2C 24 73  [dayofweek]$d,$s
010: 3D 2D 73 70 6C 69 74 24 │ 69 6E 70 75 74 0A 28 30  =-split$input◙(0
020: 2E 2E 36 2A 32 29 5B 2B │ 24 64 2E 2E 28 36 2B 24  ..6*2)[+$d..(6+$
030: 64 29 5D 7C 25 7B 27 27 │ 2B 5B 64 61 79 6F 66 77  d)]|%{''+[dayofw
040: 65 65 6B 5D 24 5F 2C 28 │ 28 72 61 6E 64 6F 6D 20  eek]$_,((random
050: 32 30 29 2D 35 2B 27 0F │ 00 0A 0A 19 27 5B 24 73  20)-5+'☼ ◙◙↓'[$s
060: 5B 33 5D 25 35 5D 29 7D │                          [3]%5])}

History:

Test script (as long as it outputs nothing the result is ok):

foreach ($d in 'Monday','Tuesday','Wednesday','Thursday','Friday','Saturday','Sunday') {
    foreach ($s in 'Winter','Summer','Spring','Autumn','Fall') {
        $o=-split("$d $s"|./weather_foo.ps1)
        if ($o.Count -ne 14) { "Wrong output length" }
        $days = $o -notmatch '\d'
        $temps = $o -match '\d'
        if ($days[0]-ne$d) { "Found "+ $days[0] + " instead of $d" }
        $m = $temps | measure -min -max
        switch ($s) {
            'Summer'{ $min = 20 }
            'Spring'{ $min = 10 }
            'Fall'  { $min = 5  }
            'Autumn'{ $min = 5  }
            'Winter'{ $min = -5 }
        }

        if ($m.Minimum -lt $min) { "Minimum temperature $($m.Minimum) didn't match for $s" }
        if ($m.Maximum -gt $min + 20) { "Maximum temperature $($m.Maximum) didn't match for $s" }
    }
}

Golfscript - 110 chars

' ':^/){*}*43845%7&5*:|;){*}*333121%7&:$;7,{($+'Sun Mon Tues Wednes Thurs Fri Satur'^/.+='day '+20rand|+5-+n}/

Here is the tests, 2 passes each to confirm randomness

$ echo -n Thursday Spring | gs codegolf-404.gs
Thursday 23
Friday 28
Saturday 25
Sunday 22
Monday 19
Tuesday 14
Wednesday 25

$ echo -n Thursday Spring | gs codegolf-404.gs
Thursday 27
Friday 12
Saturday 26
Sunday 12
Monday 27
Tuesday 17
Wednesday 21


$ echo -n Friday Autumn | gs codegolf-404.gs
Friday 10
Saturday 5
Sunday 17
Monday 24
Tuesday 24
Wednesday 12
Thursday 18

$ echo -n Friday Autumn | gs codegolf-404.gs
Friday 13
Saturday 7
Sunday 14
Monday 6
Tuesday 14
Wednesday 21
Thursday 5


$ echo -n Sunday Summer | gs codegolf-404.gs
Sunday 39
Monday 31
Tuesday 35
Wednesday 34
Thursday 21
Friday 36
Saturday 28

$ echo -n Sunday Summer | gs codegolf-404.gs
Sunday 34
Monday 20
Tuesday 30
Wednesday 39
Thursday 30
Friday 31
Saturday 37


$ echo -n Monday Fall | gs codegolf-404.gs
Monday 6
Tuesday 7
Wednesday 18
Thursday 13
Friday 7
Saturday 5
Sunday 14

$ echo -n Monday Fall | gs codegolf-404.gs
Monday 16
Tuesday 22
Wednesday 19
Thursday 23
Friday 21
Saturday 9
Sunday 17


$ echo -n Saturday Winter | gs codegolf-404.gs
Saturday 0
Sunday -5
Monday 10
Tuesday -3
Wednesday -5
Thursday 13
Friday -1

$ echo -n Saturday Winter | gs codegolf-404.gs
Saturday -4
Sunday 13
Monday 11
Tuesday 0
Wednesday 0
Thursday -5
Friday 9


$ echo -n Tuesday Summer | gs codegolf-404.gs
Tuesday 38
Wednesday 29
Thursday 25
Friday 29
Saturday 34
Sunday 20
Monday 39

$ echo -n Tuesday Summer | gs codegolf-404.gs
Tuesday 33
Wednesday 26
Thursday 31
Friday 37
Saturday 39
Sunday 24
Monday 28


$ echo -n Wednesday Winter | gs codegolf-404.gs
W    ednesday 7
Thursday 12
Friday 0
Saturday -3
Sunday 11
Monday 14
Tuesday 8

$ echo -n Wednesday Winter | gs codegolf-404.gs
Wednesday 0
Thursday -1
Friday 7
Saturday 12
Sunday -5
Monday -3
Tuesday 2

PHP, 353 319 305 304 288 characters

<?$i=split(" ",fgets(STDIN));$d=$i[0][0].$i[0][1];$s=$i[1][2];$r=json_decode('{"i":[10,30],"m":[20,40],"t":[5,25],"n":[-5,15]}',true);$n=array(Sun,Mon,Tues,Wednes,Thurs,Fri,Satur);$z=$x=0;while($z<7)if($n[++$x][0].$n[$x][1]==$d||$z){$z++;echo$n[$x%7]."day ".rand($r[$s][0],$r[$s][1])."
";}

Ungolfed

<?php
    $input = fgets(STDIN);
    $info = split(" ", $input);
    $day = substr($info[0], 0, 2);
    $season = $info[1][2];
    $range[i][] = 10;
    $range[i][] = 30;
    $range[m][] = 20;
    $range[m][] = 40;
    $range[t][] = 5;
    $range[t][] = 25;
    $range[n][] = -5;
    $range[n][] = 15;
    $days[0] = "Sun";
    $days[1] = "Mon";
    $days[2] = "Tues";
    $days[3] = "Wednes";
    $days[4] = "Thurs";
    $days[5] = "Fri";
    $days[6] = "Satur";

    $i = $d = 0;

    while($i<7)
        if(substr($days[++$d], 0, 2)==$day||$i){
            $i++;
            echo $days[$d%7]."day ".rand($range[$season][0], $range[$season][1])."\n";
        }
?>

305 -> 304: Switched the newline
304 -> 288: Uses JSON arrays instead of PHP arrays

C# 350 Characters

There must be a more efficient way than this. But here's what I've got so far:

using System;class P{static void Main(string[]x){var r=new Random();var s=x[1][2];int l=s=='r'?10:s=='m'?20:s=='n'?-5:5,u=s=='r'?31:s=='m'?41:s=='n'?16:26,i=0,z=(int)Enum.Parse(typeof(DayOfWeek),x[0]);for(;i<7;i++){var d=z<1?"Sun":z<2?"Mon":z<3?"Tues":z<4?"Wednes":z<5?"Thurs":z<6?"Fri":"Satur";Console.WriteLine(d+"day "+r.Next(l,u));z=z>5?0:z+1;}}}

Or in a more readable format, with a couple of comments:

using System;
class P 
{
    static void Main(string[] x) 
    {
        var r = new Random();
        var s = x[1][2]; //3rd char of season, first where all have unique letter

   // lower & upper limits for random, based on season
        int l = s == 'r' ? 10 : s == 'm' ? 20 : s == 'n' ? -5 : 5, 
            u = s == 'r' ? 31 : s == 'm' ? 41 : s == 'n' ? 16 : 26, 
            i = 0,
   // this line makes me cringe, but converting to an int and back seems
   // the easiest way to loop through days
            z = (int)Enum.Parse(typeof(DayOfWeek), x[0]); 

        for (; i < 7; i++) 
        { 
            var d = z < 1 ? "Sun" : z < 2 ? "Mon" : z < 3 ? "Tues" : z < 4 ? "Wednes" : z < 5 ? "Thurs" : z < 6 ? "Fri" : "Satur";
            Console.WriteLine(d + "day " + r.Next(l, u)); 
            z = z > 5 ? 0 : z + 1; // increments day, resets to 0 after saturday
        }
    }
}

D: 436 Characters

import std.algorithm, std.random, std.range, std.stdio, std.typecons;

void main(string[] g)
{
    alias canFind w;
    alias tuple t;

    auto s = g[2];
    auto e = w(s, "g") ? t(10, 30) : w(s, "Su") ? t(20, 40) : w(s, "W") ? t(-5, 15) : t(5, 25) ;
    auto r = rndGen();

    int v()
    {
        r.popFront();
        return e[0] + cast(int)(r.front % (e[1] - e[0]));
    }

    auto x = findSplitBefore(["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"], [g[1]]);

    foreach(q; chain(x[1], x[0]))
        writefln("%s %s", q, v());
}

Version with extraneous whitespace removed (which is how it comes to 436 characters):

import std.algorithm,std.random,std.range,std.stdio,std.typecons;void main(string[] g){alias canFind w;alias tuple t;auto s=g[2];auto e=w(s,"g")?t(10,30):w(s,"Su")?t(20,40):w(s,"W")?t(-5,15):t(5,25);auto r=rndGen();int v(){r.popFront();return e[0]+cast(int)(r.front%(e[1]-e[0]));}auto x=findSplitBefore(["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"],[g[1]]);foreach(q;chain(x[1],x[0]))writefln("%s %s",q,v());}
GOLFSCORE/404